I don't think it hurts to call ahead to make reservations. Not weeks in advance, but a day or two before the weekend, just to make sure they have available sites.
Plus, now that it's off season, office hours are shortened, and unless you are planning on arriving early, it's nice to know your site is all set. You can drive to your assigned site and settle the paperwork in the morning when the office is open.
I always call before, even in off season.