I usually use volleys because it conserves ammo and I have more control over my men and what targets will be shot. However, fire at will has it's advantages. Usually when I'm outnumbered and I don't want my men to die before they can at least pull off a shot.
A fine tactic I've been using is a Fire and Advance doctrine, where I let off a volley or two and make my men advance 10 paces.