I've done it a few times.
I generally go straight to the UserCP and look at the rep I gave after giving it, with no hesitation. I reread it once, think about it for a moment, then delete it if I made a mistake, a typo or something of that nature, then re-issue the rep with the corrected comment.
Sometimes I'll give a rep comment that when I read in the UserCP, I think...Gee, that's a little too harsh. If that's the case, I'll just delete it and not bother repping the post or the user again. Same goes, if I re-read it and realize it's something I'd probably report if I recieved such a comment.