Banking costs $39. If you feel like you're going to end up not using all your points in one year. You've better off borrowing during the previous year to use them up early.
I book express reservations mostly (inside the 3 month window) and I always borrow from the feature use year. The only points that I use during my actual use year are the points I use every year to do rentals with, because I have to book those 10 months out. I have 394,000 odd and 600,000 even points so I really have to make express reservations in my odd year.