Not necessary. Have an idea of what you'd like to do from the online list or the brochure, then listen to the tour spiel on videotape when you get on board. You can buy the tickets pretty much anytime including, if I'm not mistaken, via the interactive TV service on Galaxy. Stay as flexible as you can for as long as you can. You never know what sort of spur-of-the-moment opportunities may arise at a given destination.
We've never been shut out of a tour we wanted by waiting until we were aboard to buy a ticket.