If you are going even if you do not get a discount on the room(s), go ahead and book now. As BrianD posted, if a better rate becomes available, you can change your reservation.
However, just having an existing reservation is not a guarantee that you will be able to apply a discount later. To get a discount, there must be discounted rooms available in the inventory.
If you will not (or cannot go) without a discount, there is no advantage to booking now. In fact, by booking now, you are probably decreasing the chances (or number) of discounted rates being offered. Disney offers discounts when it determines that the room are not filling quickly enough with the existing rate structure. If "too many" rooms are booked, the quantity of discounted rooms Disney needs to offer will be less.
Hope everything works out for you.