Don't need to change my underwear often- I can keep using the same pair whenever I change my pants.
(OK, the comparison breaks down- but it's was a fun thing to say)
I always replace. The nuts are "pinched" to lock down, and once used will not provide the same grip on the bolt. Sure, one can use Loctite which will help, but I'd rather have the combinitation of the two.
I have seen the consequences of nut/bolt failure on a sprocket, and it's worth the several dollars to me when I replace a sprocket just for that extra-special respect for things mechanical.