Tickets do not expire until you begin to use them (or never, if you buy the no expiration option), and will only go up in price. You're better off buying them as soon as possible.
On the other hand, if you're interested in a package deal, particularly like the dining plan, you have to wait to buy your tickets when you reserve your room.
Resort rooms can be booked as room-only up to 499 days prior to arrival date. Tickets can be purchased at any time and the earlier you purchase, the cheaper you're likely to get them. Packages for Dining/Tickets/Resort are usually released with pricing early in August for the following year. JPVonDrake, a regular source for reliable info posted the packages info for '07 last year on 6 August. He'll probably do the same again when they're released this year.