I usually use a dropper when water is low and clear. Usually use some type of egg fly, glowbug or suckerspawn, small, with a nymph dropper, occasionally a small streamer dropper, like am emerald shiner fly.
Nymphs are usually a stone fly, prince, or caddis. Nagy's book explains how to fish it and rig it. Not sure why it works, but I've found that even when the steelhead are hitting only the egg fly, if I take the dropper off, they quit hitting. Also, the egg fly serves as an indicator, so you can check your drift, since I rarely use one.