I feel this should be in the realm of "recommended courteous behavior" rather than an unbreakable rule. I tend to do what Ernst suggested: If I see a bad typo within about five minutes (usually in less than one), I will correct it without adding the word "Edit" added. I agree that posters should be strongly discouraged from making changes later on that affect the substance of the post. If necessary, one can edit, adding a rephrasing, with something like this: "Edit: What I meant to say in the third line was ... "
-Eric M.
Edit: I just edited this post to add the comment that I prefer posts that are short enough so that a "full quote" isn't burdensome. Posts longer than about ten lines I seldom read anyway.