Depends very much on the singer. Some even use monitors instead of heaphones. Generally speaking you want to take out the bleaps and extemes (like guitar solo's (not that the guitarist weould be happy for you to sing over them anyway)).
I try and have some sustain (say an organ, something that's consistent throughout to rely on (and no don't associate me with traditional organ music, it's just an example (but a good one))). The beat is essential in my opionion. The volume of the sound your making (vocal in this case) varies dramatically on the singer. It depends also on the stlye of the song. Some songs you'll find yourself pushing against the song-forcing your self to be heard so lowering the volume (or raising the instrumentation) with the result of a more powerfull vocal. Tha's also a generalisation. Try and think it through. Logic is usually logical........LOL.
