I have a script written for my comic.
Dialogue is very important to me, and to the type of story my comic is. So it must be figured out beforehand so that conversations a) make sense, b) sound like different people speaking, c) tell you a little bit about that character in how they say things and how they choose to say things in a moment as well.
If dialogue isn't as important to you, that's okay! That probably makes for funnier execution or something. Spontaneous and on-the-fly. :} I'd probably do that for short stories, or a strip!