Are you planning to stay onsite or offsite? If you're booking through Disney, especially for one of the onsite properties, I prefer to get mine booked as early as possible. Rates for the upcoming year usually come out in July I believe. You can't book a DLR package through Disney with flights until maximum 330 days out, so if you're flying from somewhere, keep that in mind, too.
If you're staying offsite, I usually book my trip 3-6 months out, booking directly through whichever hotel I want to stay at. If it's peak season, definitely 6 months. Typically you get better rates the earlier you book, or if you book during a promotion. I would look for a flight as soon as the dates open up through whichever airline you use so that you can get better seats and a decent price.