Your UY does not affect when you can make a reservation. Everyone can book 11 months prior to check in at their home resort when using home resort points and 7 months prior at any resort when using non-home resort points regardless of their UY. With a Sep UY, a given set of points are valid for stays from Sep 1, 20xx to Aug 31, 20xx+1. For example, your Sep 2008 points would be valid for stays from 9/1/08-8/31/09, your Sep 2009 points would be valid for stays from 9/1/09-8/31/10. If you wanted to book a stay for sometime in May 2010, you could book that stay in either June 2009 (11 months out) or Oct 2009 (7 months out) using your Sep 2009 points.
It's best to pick a UY where the bulk of your travel occurs early in your UY, or at least within your 8-month banking window so that you would still have the option to bank any points returned to your account if you had to cancel a trip. With a Sep UY, your banking deadline would be Apr 30th. If May is your primary travel period then an April UY would be much better for you since that puts your May trips in the second month of your UY rather than in the 9th month. (A May UY would be ideal but there is no May UY.)
The question is, is that really the only time you will travel to WDW? In our case, our primary travel dates are also in May but we also occasionally visit in Oct, Dec and Jan. An Oct UY works out well for us because all of our trips fall within the first 8 months of our UY when we are able to bank points and because we do not plan to ever visit WDW in Jun - Sep due to the heat and humidity.