AP discounts can be significant, but they are something of a crapshoot, because Disney releases them only a few months in advance, and the discounts vary from year to year and season to season.
However, if you already have a room booked, and an AP rate is released, you can call back and get the discount applied to your existing booking - assuming that the discount applies to the resort and room type you have booked, and that there is availability.
Since you don't need an actual AP to book, you can book at the AP rate and get the pass itself later, either at your local
Disney Store (some but not all Disney Stores carry APs), or from WDWs web site, or when you arrive in Orlando (the Disney EarPort stores in MCO sell new APs).
So, basically, you can book your room when you decide on your dates, then wait to see if AP discounts are released that would make it worth buying an AP.
My own personal methodology is somewhat different, however. I go to WDW every year, so I buy an AP once in a while and stretch it out to cover two trips, thus justifying the cost. As long as I make 11 park days or more during the 365-day period that the AP is active, I break even on the cost of the AP before I even factor in any resort, merchandise, or dining discounts that the AP might get for me.
I have had 2 APs over the years and gotten 17-18 park days and three trips from each pass.
