Great stuff, I'm sure your post will stimulate a lot of debate. There are a couple of different ways to go. Some fish with a handful of basic patterns and shades in various sizes and cover the bases that way.
Some will get into some general themes with a few specific styles of flies and variations in size and color for specific hatches. 9which I think you’ve suggested here)
And then you can really go nuts tying for specific hatches with variations in different styles based on the species' habits and life cycle as well as size and color.
I try and split the baby between having relatively few different "styles" of flies with variations based on size and color, and trying to know a little bit about each of the hatches to make more effective imitations--- not so much in terms of realism, but in terms of how and where they are designed to be fished. I tie a lot of different stuff for FW and SW, and don’t have the advantage of a home stream, so my flies tend to emphasize easy ties that I can crank out and that use inexpensive materials (in theory anyway)
In addition to the westfly entomology pages, this might be a good resource for you (Steve’s page has a ton of useful info for fishing all over the Sierras):
Troutnut is also a great site for behavior on specific critters and pics of naturals so you can have a good place to start for shades and colors, though most of the info is Midwestern and Eastern oriented. Over time you might notice different shades on the naturals out there:
Troutnut.com Fly Fishing for Trout
This will give you a good idea which pattern styles might be most effective, what part of the stream to look for them (riffles, slow water, silt or cobble bottom, etc), water temps they emerge, and what time of day different stages of the hatch tend to occur. In general, you can get away with a lot more impressionistic patterns in faster water. A few more variations in slower water hatches might be useful.
Just as an example, here in the east we have a couple of hatches- Quill Gordons and Isonychia (probably very similar to your mahogany duns if you get them), where they deviate a bit from the “normal” mayfly life cycle. Quill Gordons nymphs are fat and tend to mass downstream of rocks in fast water, then “emerge” on the bottom, so heavily weighted nymphs and a winged wet fly is more effective than typical mayfly nymph and emerger patterns. In the case of Isonychia, the nymphs for the most part crawl out of the water onto rocks, so emerger and duns patterns are typically not as important. Also in the case of Isonychia, the spinner form is very important, and the females skitter around on the surface laying eggs. A hackled pattern, the Dun Variant, using over sized hackle can be very effective for this hatch. You may also find that spinner patterns (rusty spinner in most cases) or cripples can be very important for some hatches out your way. Some hatches the nymphs swim to shallow water, so having some unweighted nymphs is a good idea. Spinners can look dramatically different than the duns.
So, if you’re going to tie to match the hatch, some simple variations on the general theme might be helpful as long as you’re going to tie them.
I'm no expert, I'm sure others have better solutions, but just some examples that may apply in some cases for your hatches:
Nymphs- I mostly use dubbed body nymphs in different colors to imitate the naturals, mostly with beads. Depending on the specific mayfly, nymphs are either fat (GRHE) or thin (PTN). I always use very sparse dubbing on single thread in place of PTNs, and for fat ones, take several passes. Wire ribbing holds everything in place.
Emergers- I use a nymph (in nymph body color) with a ball of poly, or a poly wing on a dry fly hook (some mayflies like Blue Quills here pop their wings just before the surface), for larger drakes a dubbed body with a short wing of snow shoe hare’s foot. I also mix in some deer hair emergers using the dun’s color for the dubbed body on a curved light wire scud hook (TMC 2487) for drakes and other medium-larger patterns or substitute CDC for deer hair on smaller patterns, usually with an Antron shuck. I mostly do this just to mix it up a bit. A lot of the water I fish gets a lot of pressure and is fairly slow moving with wild fish that seem to look things over and count thread wraps.
Sometimes a change up seems to help, sometimes it doesn’t. Mostly it’s an excuse to try different patterns. If you get Western Green Drakes, a dubbed body nymph with short snowshoe hare wing or a Deer Hair Emerger could be killer.
Duns- I really like Sparkle duns and fish them a lot, and usually mix in a few Comparaduns (split tails instead of shucks). The Sparkle duns do double duty as both dun and late stage emerger, and can also do a passable job as a spinner. They can also be fished wet by tugging them under. As Dshort mentions, you may want some hackled dries for some hatches too for fast water (clinger mayflies). They’ll float better and be easier to see than sparkle duns and thorax style dries, so you may want to add some. Not needed for all mays- Callibaetis out your way hatches in lakes, so a thorax or sparkle dun might be perfect for them. I typically use a wing of natural or dyed flank, single clump not divided on hackled patterns. Some mayflies Tricos and our Eastern Hendricksons have different look for males and females, so that might be another thing to think about.
Spinners- For some specific hatches this could be important. You might want to add a spinner pattern dubbed body, poly wing is easiest, but there are a ton of variations using hen wings tied spent, cdc, hackle, quill bodies etc. I often tie them in the round with a hackle collar (no wing) on extra light wire hooks, and skitter them on the surface, or trim hackle top and bottom on stream to fish in the film. Luckily a Rusty Spinner doubles for many mayflies including most BWO’s, PMD’s, Red Quill etc.
Cripples, I don't tie many intentionally (though some seem to come out that way)..., but for some hatches, like PMDs these can be very important. A dry fly body with CDC wing, no hackle, is great for these. I believe the Deer Hair Emerger also fishes for these as well as emerger. If you wanted to experiment with just one, a Deer Hair Emerger style tied with CDC instead of deer hair sized and color for your PMD would be good, and if it produces you might consider it for BWO/Baetis.
Larvae- the naturals live in cases for the most part. For the east, I only use one pattern, the Green Rock Worm for Rhyacophelia, which is a free living caddis larva (without case) found in riffles. This might be a good one for you too. If you have a lot of cased caddis that use metallic sediment for cases, a brassie might be good as well. I generally tie them just in the largest size of the hatch. So if a given hatch is listed 12-16, I'd tie them in 12.
Pupae- I like both a deep fishing version with a bead or added weight and one that fishes higher up. A Deep Sparkle Pupa Green with Brown head works well here, you might also consider a size 8 orange with brown head for October Caddis if you have them. A Z wing caddis or Tabou Caddis for fishing higher (though a simple softhackle often does well too here.) I just shoot for the middle here, so if the hatch is listed as 12-16, I just tie them in 14. Green with brown head covers a lot of bases here, but it could vary a bit depending on the caddis you have.
Adult- The EHC is great in different sizes and shades. I also like to have a few with a heavy hackle collar (Fluttering Caddis style) to skitter. I also use X Caddis and Elk and CDC patterns a lot because they’re so fast to tie, and very good for slower water (not as easy to see on fast water though). If you can find which of the caddis hatches lay eggs on the surface, you might want to tie a few flutterererers mixed in with the EHC they use the same materials and tying techniques, just put a sparse wing on further back, without the “head” of trimmed butts and wrap the hackle behind the eye instead of palmering it. You can fish them the same places and same way as the EHC as well as skittering them. The EHC hackle is typically undersized at 1.0x (=) hook gap. For fluttering caddis it should be conventionally sized at 1.5x hook gap.
Diving Caddis- some adult females dive to the bottom to lay eggs. A size 12 or 14 Partridge and Green, soft hackle, a great all around searching fly in it’s own right, also covers a lot of situations and can be very effective. Again, knowing which caddis dive to lay eggs will help here.
For the nymphs, I tie small ones like the winter early black/brown stones as thin bodied dubbed bodies Hares Ear style for the nymphs, and use dark EHC for the dries. These stonefly nymphs are relatively thin bodied, at least here in the East, compared to the big fat ones that pop later in the season. The Prince Nymph is also a good searching pattern and doubles for smaller stoneflies (and can stand in for some large dark bodied drake mayfly nymphs).
But for bigger stoneflies, I use A.K. Best style “Big Ugly” and Golden Stone nymphs found in AK’s Fly Box. They’re a fast tie, I use biots for the tail, skip antennae, and use one wing case instead of the two found on the naturals- doubt if trout can count anyway. You can lose a lot on the bottom if you fish them where they should be, and they’re mostly used in fast water so trout don’t have much time to look them over. I don’t waste too much time on realism (not that I’d be any good at it anyway).
I use EHC for small stones to 16, Stimis for the medium- large dries up to 8, or big deer/elk bullet head for really large ones.
Grass hoppers. I like stuff that lands with a big splat, floats like a cork and can be twitched, and the easier to tie the better. Ants, just 2 balls of dubbing and hackle in the middle.
Just as a general thing, size can be very important as Eaglesfan and JDorsey point out. Often the naturals appear to be bigger than they really are when trying to match on the stream, and also fish seem to be more apt to reject a larger fly than the natural fly more often than a smaller one. So for hatches where there is a range of sizes, say 12-16, I tend to tie more of the 14 and 16’s than 12’s. The 14’s will fish for the 12’s and 14’s, and the 16 will fish for the 14’s and 16’s. This seems to be true for dries anyway, but you can often get away with larger wets like softhackles, perhaps because they tend to be tied on shorter hooks, or have shorter bodies.
When I tie in the winter, I try tying up for each major hatch in the order they occur and attempt to end up with at least 12 for each category nymph, emerger, and spinner, and 24 for duns mixing up styles for each of the hatches I plan to target.
It sounds like more than it is, as you start knocking them off, many of the nymphs and spinners you tie will fish for different hatches too, and in some cases it’s just a question of topping off what I have already. During the season, I try to tie replacements for stuff that seems to be working, new recruits based on eye candy seen in shops or this board, and crash test dummies for experimental flies if I have some kind of brainstorm (that usually turns out to be just a drizzle).
Someone a lot smarter than me once said: "90% of what trout eat is brown and 3/8" long." Keep this in mind as you go into tying different patterns. But I tend to over complicate things...