Haha, I do that, too! Well, maybe not whole paragraphs. But awkward sentences sometimes bug me enough to earn re-writes.

I always put a note at the top ("Edited by ferdelance to correct [minor spelling/grammar/misspelled name/whatever I did]"), though, beneath the title and author info, to be polite.

Yes, I know. "Polite" to pixels on my hard drive that the author will never even see. I can't even explain it to myself. It just feels wrong not to.