We've always bought our AP vouchers in advance (online), but as Rob says, there's no real advantage in doing so other than possibly avoiding any interim price hikes. Next time we need to renew, I don't think we'll bother.
Whichever option you choose, you'll need to take along some photo ID (we use our passports).
You <i>can</i> get the Disney Club discount (5%) on advance purchases, but they don't make it very easy.