Do you think you might ever want to start your vacation at the end of August? If so, a September use year means the points used for those August nights would almost certainly be lost if you had to cancel, because they expire at the end of August. Also, you cannot book online across use years. You would have to call MS to finish your reservation.
Will you be seeking something hard to book at 11 months - i.e. standard studios at BLT or BWV, value or concierge at AKV, or etc)? Those reservations are often "walked". If your desired reservation is in September, walking may not be available to you (because you can't walk effectively across use years and you can't start a walk until 9/1). Others who also want those rooms will have started their walks ahead of you and blocked you from starting yours.
August or June would be good use years for a pattern of September travel. If you have to cancel, the banking deadlines for those use years and point expiration dates still give you a reasonable time to use the points.
Good luck wth your decision.
