Should you edit old entries to correct mistakes or should leave it as a snapshot. If a blog is form of a time capsule, each entry should be considered sealed for prosperity. If a blog is consider a living resource, it makes sense to trim and prune it much like you would care for a garden.
If I'm just fixing typos or punctuation, I'll do it silently and not worry. If I add anything that substantively changes the meaning of a post, I'll label it. I try never to delete anything substantive. Attempting to un-say something by deleting it is really just a case of hiding the evidence. I'd much rather correct myself out in the open than pretend I was never wrong in the first place.