Conventional wisdom has it that trout can be very line shy, and that one should use very light leaders and tippets in clear, heavily pressured water. The problem, of course, is that 6X to 8X tippet has a breaking strength of less than 2 pounds, leaving one little margin for error when you hook a fish.
I tend to go against conventional wisdom. I never use anything lighter than 6X, and seldom go below 5X. I fish both some heavily pressured public rivers in Missouri and several famous streams in Montana, including the spring creeks on the Yellowstone near Livingston (Armstrongs, DePuys, and Nelsons).
Here are my thoughts on the matter. If trout can distinguish a tiny midge from another bug and key on the midge--and refuse your fly if it's not quite good enough as a midge imitation--their eyes have to be good enough to see your line, no matter how light it is. So the line can't be a huge turn-off to them. Their brains are about the size of a pea and they don't think in the way that humans do. I'm pretty sure they don't think, "Hey, there's a nice looking little trico, but it has a line thingy hanging off it, so I don't think I'll eat it."
So I don't think that they pay much attention to lines the diameter of the tippets we use. I think the lightness of your line should not be determined so much by the clarity of the water and the wariness of the fish, but by how that line affects the drift and movement of your flies. 3X tippet is going to really affect the drift and attitude of a size 22 dry fly (even if you can fit it through the eye of the hook). But it won't make any difference to the movement of a size 8 woolybugger. So I mainly match the size of my tippet to the size of the flies I'm using, not the clarity of the water, and I go as heavy on the tippet as I can and still have the flies acting and drifting realistically. I use 4X to 6X mono for dry flies, 2X to 4X fluoro for most nymphs, and 0X to 3X fluoro for most streamers. And I catch fish wherever I fish.
I also feel the trout don't "See the line" they see a #22 midge floating incorrectly in the column they are eating in. One of the many books I read (i forget which one) said if you sit down at McDonald's and your order of fries moves across the table you wouldn't eat them because that isn't what an order of fries is supposed to do.
If you get a chance stop at "Readings" and get Charley talking about tippet he can talk hours on why in clear slow water you need the bug to float like a bug floats.
As far as lbs... Airflow 7x = 3lbs and Seaguar Grand Max 7x= 2.6
Both are expensive but this is all I use anymore. I think the Seaguar is the best in "Gin Clear" water at 7x.
Plus I am a fan of Doubles (anything) no mater what and I believe the fish will maybe back away from something wrong with the first fly but will grab the second.
6 weeks ago at Bennett below the apron the fish moved up in the column to feed. I put on a #12 attractor fly (looked like an elk hair caddis with bright body) and a #18 griffets gnat and had a ball. The water was clear but watching the trout they would run to the attractor back away only to hit the gnat. This was almost every cast for 45 to 60 minutes. ( I only wish I could have landed the fish I tried to hook) :^)
Again.. I agree smallest tippet possible for the conditions.
i pretty much just always use 6x tippet but i do enjoy fishing with the lightest tackle i can get away with and ive only been spooled 2 times in my life. once by a dolphin and the 2nd i could not get the fish to turn around with my heavy rod and 20# line. with trout ive hooked a brown i was told was 5-7lbs but it bent my hook straight with 6x tippet. just gotta set the drag a bit lighter.
as far as them seeing the line. i doubt they can but i do imagine thicker line doesnt allow the fly to move as natural as possible, and if its not a natural flow the fish wont hit it. sure the fish has the brain size of a pea but they do have instincts and react to certain things without thinking.
I very rarely go below 5x tippet for trout; 4x whenever I think I'll have a good chance of hooking larger fish.
With smaller fish, I will go to 6x, but only on a very rare occasion; like fishing with Neversink for Yamame/Iwana in the skinny, gin-clear waters of the western/central Japanese mountains.
For the most part, my "drag" is the tension that I apply to the line with my left (stripping) hand as I'm either bringing a fish in or letting it run out. After a few fish, you tend to develop a sense of what kind of tension will break your tippet and what won't.
With constant tension, 5x tippet will take a considerable amount of pressure before it parts. With instantaneous changes in tension, you'll get a lot more broken tippets. That's one of the reasons that I don't let my fish run on me; unless it's a really big one and I don't have any choice.
If you ever see a trout surfing its way across the water and toward a landing net, then there's a good chance that I'm fishing right near where you are.....
I go down to 7x on some spring creek trout streams, if I'm fishing small flies, especially. I will not use 6x or 7x on flies larger than a 16 - makes no sense. For trout and steelhead fishing, I have noticed a difference in my success rate based on tippet diameter and when I get below 4x I always fish fluro. YMMV.
I was fishing this weekend with my 1 wt and I usually leave it on 7x tippet since it primarily used for one thing size 18 dries and thats it. Finally after snapping the line a few times on the hook set ( I know I know dont need to set the hook like a bassmaster) I finnaly cut it off and went to 6x. Between my skill level and the minute difference in presentation I will be sticking with 6x only. I might use 7x for just the dropper but I find I dont set the hook as hard when they hit the dropper.
i used to use 6x for fishing for pan fish, but a handful of break offs from bass hookups made me switch to 5x or 4x, depending on the water.
the only time ive used 7x was in a lake that i was sure held no fish more than 10" or so.
Consider the rod you're using and your technique. I use the lightest tippet I think I can get away with. If I am on light 6 or 7X tippet in slow, clear water I know the rod I'm using is forgiving and let it absord the little shakes thrown by the fish. I also get the fish on the too expensive 'line holder' as fast as possible to let the drag do what it is built to do.
Suprisingly, I manage quite a few nice fish on fine leaders but again, I only go that way when it is really neccesary.
Think about it, to be a member of the 20-20 Club you have to be able to stick the tippet through the eye of a hook size 20 or smaller.
including the spring creeks on the Yellowstone near Livingston (Armstrongs, DePuys, and Nelsons).
There's a good book on the Spring Creeks (Buzz has it at his shop on DePuy), explaining his tippet choices and how it affects his drifts. With a light rod he fishes 7x and 8x..
Whole different way to look at things....