Use yer only matters if you regularly travel towards the end of the use year and have to cancel the trip after your banking deadlines have passed. (If you don't have to cancel, then use year really doesn't matter at all).
For example, if you have to cancel an October or November trip and have a December use year, you would have to use the points before the end of November to avoid losing them. If you were able to cancel the trip before the end of September, youcould bank up to 25% of your annual allotment.
Anyway - for those who think they may have to cancel a trip, December use years work well for those who regularly travel in December through June. (As long as you cancel your trip more than 31 days in advance, you can bank up to 100% of your annual allotment if you do it by the end of May.
If you don't have a regualr travel pattern, use year probably doesn't matter. Many families' vacation patterns change over the course of their membership, so what starts out as a "perfect" use year could very well end up as a "horrible" use year or vice versa.
Keep in mind that reservations/points can be rented or gifted if circumstances prevent using them as originally planned.
Best wishes -