You know, even though I would probably use an edit function if there were one, I'd prefer that there not be one. Anyone who's ever written anything always looks back on it and sees stuff they'd change. You could drive yourself crazy changing little things about the wording and such ad infinitum. I'm glad that there's a definite cutoff for editing your comics. Clicking "Save" is like sending a book off to the publisher or slipping a term paper under the professor's door. It's probably not perfect, but for better or worse, it's done.
On the subject of grammatical errors, I've found that they can be easily prevented with a little proofreading beforehand. In all the strips I've written, there's only one error I'm aware of as far as grammar, spelling, and typography goes, and that was because I was testing to see if HTML was supported.
Another point worth mentioning about editing is that you could theoretically use such a function to change a strip entirely, so that it had not one element in common with the original work. I don't think that's a good thing.