The rule of thumb is to try and travel toward the beginning of your UY so you have the flexibility to change or cancel a reservation (more than 31 days out--less than that and there are different rules).
For May travel, April UY is ideal (since no May UY exists). If you change your vacation to less points, or cancel the trip, you still have quite a bit of time to decide what to do. Since you can bank points during the first 8 months of your UY, you can reschedule the trip, or bank the points.
I think it especially comes in handy if you borrow points and have to cancel. It still gives you almost the entire UY to rebook your trip.
As PP said, UY plays no role in when you can book a trip. If you want to travel in May 2011, you call in June 2010 to book it at your home resort (or in Oct 2010 for all other resorts). As long as you will have points in your account that are valid for a May 2011 trip when you travel, you will be all set.
Good luck!