Use year doesn't matter at all for making reservations - you can always do that 11 months prior to check out for your home resort and 7 months prior to check out for a non-home resort.
Use year is just the month that your annual allotment of points is renewed and it also determines your banking deadlines. The latter could be important.
When does it matter? Only if you frequently make and then have to cancel a reservation that occurs late in your use year. For example, if you frequently travel in October or November, a December use year could be a problem if you have to cancel a trip. That's because even though you get the points back from the cancelled reservation, you only have until Novembr 30 to reschedule your vacation or rent the points. If you can't do either of those things, you lose the points.
If you usually travel in January through May, it's not the same problem. If you cancel a January - May trip and couldn't reschedule before the end of the use year (November 30), you could just bank the points. With a December use year, you have until the end of May to bank up to 100% of your annual allotment.
For those who don't have to cancel a trip occuring late in their use year, use year month makes no difference at all.
Keep inmind that over the course of your membership, travel patterns will change. What might start out as a "perfect" use year, could certainly end up as not so "perfect".
Best wishes -
P.S. Obviously, "late in the use year" varies depending on your use year month. Ours is August, and I'm quite happy with it. FWIW, we just took the use year being sold when we purchased. We had no idea at the time that use year would ever matter. So far, it hasn't - we've been fortunate and have never had to cancel. Since we prefer to travel in the fall & winter months, August will work well for us anyway - at least for now, LOL.
