You would want a Use Year that falls in the first six months of the year, if you are sure you will always travel in June or July. I don't think there are any January or May Use Years, so February, March or April will be your best bet.
That's because you can book 100% of your points in the first six months of your Use Year. If for some reason you realized one year that you couldn't go during your usual June/July time frame, you'd still have until the end of July to bank all of your points into the next year (assuming a February Use Year, because six months from February 1 would be July 31.) For a March Use Year, your 100% banking deadline would be August 31, and so on, counting 6 months from the first day of your Use Year month.Then you'd have the pleasant task of deciding how to use twice as many points the next year!!!
Use Year can be confusing, because it has nothing to do with when you can make a reservation. Use Year is just the time each year you get your "new" points. You can make a reservation 11 months ahead for your home resort and 7 months ahead for any other resort, as long as the points will be in your account by the time you take the trip.
Something else to consider is Spring Break. If you think that at some point you will have children and want to take a Spring Break trip, those dates are usually in February or March, so an April Use Year might not be such a great choice. I have a March Use Year, and that covers Spring Break and summer trips. By the time my 100% banking window has passed, at the end of August, I have a very good idea of my travel opportunities for the rest of the year (and I have usually already used my points!!)
Hope this helps!!!