As far as learning to cast, a 5 wt will be considerably easier than an 8 wt.
As far as what line wt of fly rod for what fish species, much depends on the skill of the angler and the rod action as well as the fly line rating. For example, Gary caught this 14 lb 11 oz brown trout on a 9 ft 5 wt fly rod. Notice how thin the rod look in relationship to the fish. It is much bigger than most stealhead that you will catch.
Gary Borger » Blog Archive » Fall Salmon and Browns 2013
A modern graphite fast action fly rod will be able to handle 2 line weights higher for casting. So a fast action 7 wt rod will handle either an 8 or 9 wt fly lines. I've cast a 9 wt fly line with one of Gary's 7 wt fly rods and it performed fine.
The question then becomes will the 7 wt fly rod be able to tame a steelhead. I think so. Gary hooked Chinook Salmon with that 5 wt but was unable to put enough pressure on the fihs. If he had a 7 wt, I pretty much guarantee you he would have landed them.
My view is that an 8 wt fly rod is too heavy for bass unless you are going to catch some brutes. I also think a 7 wt should be able to handle most steelhead.
The question becomes how often will you really need an 8 wt?
So my vote would be for a 7 wt. but you need to consider if you need the fish fighting ability of an 8 wt, and is sacrificing the ease of casting and pleasure in fishing a lighter rod worth giving up for the times you will really
need the 8 wt?