If you really want September or October, just ask your guide - others have requested specific use years and received them. Unless they have already sold out those months or it somehow impacts your incentive (and then the guide will tell you), I'm sure you can get what you want.
FWIW, we have an August use year and it works just fine for Fall & Winter travel. You have until the end of January to bank up to 100% of your annual allotment. Since you realy need to cancel at least 31 days in advance (to avoid the holding account), an August use year works well for trips scheduled August - February.
Also, try not to worry too much about getting a "less than perfect" use year. Your travel times are bound to vary over the course of your membership. What is perfect right now, might not be so perfect later.
Best wishes -