During the first 8 months of your UY, you can bank your points. When you travel during those times, you have a lot of flexibility to adjust or cancel your trips.
I own June and travel in August. Last summer, I could make changes to the reservation right up until 31 days prior without worrying about the changes taking less points as I was traveling only 2 months in.
Plus, if you borrow points for a trip and end up having to cancel, you must use the points by the end of your UY. If you are traveling late in the UY, you are not given much time to use them up.
When I was first bought my VWL contract, I bought with Oct UY. I then realized how horrible that would be for my August trips since it would only leave Sept if I had to cancel a trip. I teach and can never travel in September, so there would never be a chance for me to use those points. I sold and ended up buying in at BLT with a different UY.