I fish a lot of small streams, at least what we consider small in SE Idaho and Western Montana.

I have a distinct preference to stay out of the water whenever possible to minimize my effect on the streambed and residents, meaning aquatic insects and their habitat.

Having said that, in the final analysis, my approach is dictated by casting angles and how best to get the fly to where I know ( sight fishing ) or believe ( fishing the water ) the fish are. If that can be done from the bank, that is where I'll be. If it requires wading, that's what I'll do. Streamside foliage and overhead cover, wind speed and direction, and current speed and variety all come into the mix.

John