I think the only problem would be if you reserved your week in December and then needed to cancel. If you had borrowed any points to make the reservation, they could not go back into the year you borrowed them from. You would have to make sure that you could book another trip before your next use year started.
Our use year is June and if I book for spring break, I have to hope I don't have to cancel because, with four kids in school, I can't plan another trip before June unless I take them out of school. Of course, that is if I have to cancel after my within 30 days of our arrival.
Curiously, the guide book says borrowed points can't be returned to their use year, but on the page about cancelling reservations, it says that if you cancel at 31 or more days, all points will go back into their use year, but they don't say that that doesn't apply to borrowed points.