Most of the time when I fish small streams I will fish 6x for dry's, unless I am tossing big stimulator and madam X's. I will go up to 5x. If I am fishing a double tandem to a small midge or nymph I will sometimes drop down to 7x, but not often. Usually I will if I think the trout are being picky in ultra clear water, and I am worried they can see my tippet.
I guess most the times, beyound fly size, it is how hard the fish are to get a bite out of. If they are heavy pressured fish I tend to go small flouro. If I am fishing water that doesn't get a lot of pressure and has eager to bite fish, I may tend to fish a little larger tippet and mono, cheaper!
Oh yeah, one more thing. If I am fishing a long slow super clear pool, I will go small tippet (6 or 7x) I have been known to tie on 8x before, but that is a test in futility sometimes. If I am fishing choppy water or around plung pool and falls, I will use a larger gauge tippet.
And one more thing beyond the one more thing above. In extreme situations (like real deep dark pools), if I am dropping a size 6 or 8 Stonefly that is weighted up nicely, I will tie on 3 - 4x flouro, just because you never know what will bite the other end.
You, know... after typing all of that. It maybe why I find myself tying more knots than fishing on some days.
May have to streamline this thought process. May have to do like Russ... pitch 5x and let it roll.