It depends entirely on when you like to vacation. Ideally, if you have a December Use Year, you will POSSIBLY want to complete your vacations by August 1st. Why? So that, in case you need to cancel or change your plans, you can just Bank them. That opportunity to Bank will end at 4 months before the end of your use year. In other words, you can Bank up until July 31st, but after that you can only use them, trade them, rent them out, or whatever.
We often travel in early to mid October, so my first contract was a September Use Year. But since then, I have added contracts in February, March and more in September. I often Bank points, but if I'm not sure whether I will be using them, and I think I might, then I don't bank them, and if it becomes clear that I won't be using them, I rent them out. Of course, the alternative is to just Bank, for convenience reasons, and then, if you decide to take a trip in the last 4 months of you use year, just Borrow the points.
Anyway, there IS NO one, BEST Use Year. It is just like everything else. Different Strokes for Different Folks.