This spring was the first time we booked a room before the discounts were released. Sure enough, when discounts were released, our room type/resort wasn't eligible and we ended up changing it (before it was all cancelled that is). For our rebooked trip this August, we did what we normally do and "shopped" the best deal based on the discounts. I like that better--that way I know I have the best combination of resort we like, room type, and price.
If there is only one resort/room category you want, then book ahead; otherwise you might as well be flexible.