There's really no down side to booking in advance. It's fast and easy, and if you need to cancel, the tours are refundable outside of 24 hours before the tour departure.
If you're sure about which tours you want, I'd book 'em, Danno.
Booking them aboard the ship is easy, too, if you know what tours you want. You just fill out a form and drop it at the Shore Excursion desk. Or, if you're not sure which tours you want, you can go to the shore excursion talk that's given on the first day, but then you run the risk of everyone wanting the same tour, and not getting the tour/departure time you want.