The dialogue and misc. text can actually get put in anytime after I'm done with Illustrator. Lately I've been doing it before adding backgrounds, just so I can get an idea of where it'll be covered up by bubbles.

Any last minute revisions to the script can be done as I type out the dialogue. Usually it's nothing major, maybe just changing a word or two to make something easier to understand or to fit better into the speech bubble. At the same time, I also will add a layer for textual sound effects or certain visual effects.