As Sandi put it, a September UY would not be "ideal" for June vacations. August would be worse, October not so bad, etc.
However, UY is just one factor you have to deal with. Where UY really comes into play is when your plans change. You can bank points only during the first 8 months of your UY, so for a Sept UY, your banking deadline would be April 30 as the poster above stated.
So...IF you booked a June reservation and then changed your mind on May 1 or later, you would not be able to bank those points. You could still use them up through the end of August, but you couldn't bank them.
Whether or not that is a problem for your family depends. It's certainly a factor you have to be aware of, but you'll have that issue the last four months of any UY. The key question is really, how solid are your vacation plans by April 30?
For example, we have an October UY. My two adult daughters
always vacation during the summer (both are currently on vacation, one at VB, the other on a non-
DVC vacation). In five years -- 29 trips and counting by three families -- we'ver never had a problem with vacationing late in our UY.
In fact, my daughters are rather notorious for not making vacation plans until pretty darn late in the process. So we bank points prior to our May 31 deadline and then borrow points for their summer stays. As long as their plans remain firm, we not only don't have problems, we also don't forget to bank our points.
IMHO, UY is not the bogeyman we sometimes think. It is something that has to be understood and managed.
Also, over the years, our vacation patterns can change. What's true today may not be true five years from now.