These are the principles I’m currently trying to follow:
Treat writing a blog like any other peice of published writing: spend some time on it, re-read it, correct it, don’t publish it till you think you’re done. Use the draft feature of your blog software if you need to.
If you spot a spelling error or any other minor non-semantic blemish in a posting, it’s OK to correct that at any time.
Substantially altering a posting is like trying to alter history: it’s bad. At a minimum it might be confusing to your readers. At worst, you are deliberately altering what you’ve said. Making the alteration clear (the previous version in brackets, say) is probably OK, but it might be better to add your correction as a comment. I sometimes make an exception to this rule for the most recently posted entry, but only for minor semantic improvements.
Of course, ultimately, my blog is my own, and I do have the right to do whatever I like with it. And if I decide I want to alter an entry (I’ve libelled someone, or I’ve misunderstood something which has substantial repercussions on my reputation), I probably will. But by and large, I will try to abide by the principles above - just because it seems like the honest thing to do.