The earlier you book, generally the best price you'll get. If Disney does by chance drop the price, you can get the better one. Reservations are open through May '05 right now.
If you are a family of 5, your room choices are a category 4 or a suite, or connecting rooms in categories from 10 up to 4. The 11 and 12 rooms do not have doors connecting to another cabin.
It is great fun and a little overwhelming at first to plan after you have mastered the parks. Disney actually has what seems to be the least convoluted room layout and charts...everyone else has a zillion categories and another zillion or so sub categories, all with different pricing structures.
DCL makes it pretty simple to understand.
Fall and winter, except holidays, will be less than summer/ holidays. At the official website, disneycruise.com you can get a quote right there to see how the prices change throughout the year.