I think that revising before print is a good thing. I've revised the two Voles of the Dusk stories for print. Differences between the print and the online versions of a story are generally acceptable.
Revising an ongoing webcomic, though, opens up a whole other can of worms. Will you just be editing and redrawing a few things, or will you be significantly altering the story? How will you express this change to your readers- ask them to go back and reread the edited sections, include a textbox saying "Retcon in effect: before becoming a monk, Pang was briefly married to Joan Collins", or what? There's also the danger that by going back and fixing just a few things, you'll have opened the Pandora's Box of neverending revision. I know some people over at ComicGenesis whose story never advances; they just keep redrawing the first 30 pages over and over again.
Let me tell you about what I'm doing now. I haven't been very happy with the way my "Darcy Generic: Martyr w/o a Cause" story is going. I love the characters and the situation, but it's not connecting with readers and I know it can be done better. Starting from chapter 4 the entire series is going to be reimagined; the format is going to change from a regular comic book style to a much denser serial style. A few of the story elements are going to change. I'm going to try to add more paranoia and darkness, since that's obstensibly what the story is about and I think Darcy's midlife crisis kind of derailed the first few chapters. I'm not going to go back and redo the first chapters, though- at least, not yet. When the time comes to assemble a print collection I'll redo those chapters entirely. Any changes to the backstory that need to be addressed will likely be added as footnotes to the comic.
Doing things this way doesn't take a lot of commitment. If I do chapter 4 and decide that it's total crap, I can go back to the old style in chapter 5 and haven't lost much effort. Or, I can start something completely new with chapter 5. The fact that the layout and pacing is going to change will serve as a visual cue to the readers that other things have changed as well, so when I explain the retcons they won't be so much of a shock.
if you sat around with a script like you would a prose story, you'd never get any drawing done.
That's how I used to write comics. Part of me, the part with the bachelor of arts in dramatic literature, thinks that's how I
should write comics. I don't know the answer to this myself. It used to be that I never had time for drawing; these days it seems like I never have time for writing.
I've rambled on for far too much, but hopefully some of it will be useful.