If you book now, you will lock in the current pricing and secure a room. However, if you are traveling in September chances are there will be a promotion available for that time. If there is an you apply the promotion to your current reservation, the prices will be adjusted to the prices effective that day (not the previous prices locked in). So, really the only thing you would be doing now would be securing a room.
If you be traveling during a time when promotions were not available, say Thanksgiving or the week after Christmas, then I would say book now to lock in current rates AND secure a room.