I still re-read books I wrote twenty years ago and think "I shouldn't have used that word." I can waste hours going over what I already wrote, but fortunately, if the Muse is on the job, she will take me by the ear and drag me to the foot of the text to write the next bit.

A certain amount of revision is good: usually what I first wrote needs a lot of trimming and polishing. (And cutting down, I use RAH's method, write everything that comes into your head and then throw out half of it on the rewrite.) As for when to stop, deadlines are your friend. Eventually, if all else fails, you can tell the editor "yes, it probably would be better for another rewrite, but there's no time."