Finally, from experience I've formulated a rule of thumb:
Last-minute edits on a work that has been in progress for a long time, or a published work about to be republished, are mistakes.
Last-minute edits on a work you have very recently completed are likely to be good edits.
If one is "in the zone" of creation on a particular piece, ideas for revision rise from the same pool of thought, or, as they say, "are organic." After that work has been completed and set aside and another work commenced, that same zone simply never comes again. The author has changed or grown and can't step into the same work twice. I have longed to add a new insight to an old work, and did, and now that extra sentence in there jars me, and I am concerned that it might jar readers just as much.
One's old stories and poems can be rewritten or refurbished and turn out very well, but not if rewriting or additions take place at the last minute; say, a few hours before a contest deadline. It's like sewing a new sleeve on an old shirt. It might fit but it will be a slightly darker shade or nap or texture, and even if no one else knows, the author does. Is it too small a thing to care about? Not if you care about craft.