I play bass and drums in two bands with the same frontman. I pretty much do whatever I want, but quite often I get "oh, I wanted it to go like this". Then it's down to me to say I like it or I don't. If I don't want to do it his way it can get a bit awkward. I think I generally back down and go along with it, especially on drums which i'm less experienced on.
I think the generaly rule for our band(s) is: If it's significant (a chord, a route note, a rythm) then any member can comment on it. If it's insignificant (fills and solos basically) then it's down to the individual.
If you dictate to a guitarist how to play a riff for example, it could end up sounding like you playing, rather than him.