I would suggest keeping them (albeit in a smaller font, and not have them show up on the editor module -> which lead to enless lists of "last edited on ...."), but maybe only either after 1 hour of the original post or if it is not the last post in a thread.
This way, one can amend a post he's made immediately after submitting it (any TBH, who uses the Preview feature all the time to make sure everything is correct), and it doesn't immediately show up as edited. But if there are changes to older/earlier posts, an edit mark would certainly help to put things in perspective and not have answers to posts that point to nowhere...
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