Depends on the comic you're making, but for mine I have to have a very detailed script to keep track of everyone's movements, things going on in the world, and making sure that I'm not leaving gaping plotholes. I don't like doing it chapter by chapter, either. My whole script is finished (and yet I still find myself editing stuff out once it hits the page, so...it's finished to the best of my ability, at least)
Drawing takes so damn long, that the last thing I want to do is make a chapter I'm gonna have to erase later because of plot issues. I want it to be as solid as possible so I can just start pushing this art out rather than spending time trying to work through writers block while under a deadline.