I won't use anything during the winter months. This is the ultimate test you can give to your wax build-up. I would've usually accumulated an adequate amount of a wax build-up so as to float through the winter months. If you've taken great care in building your wax layers properly, then it will last you through the winter, regardless of whether its covered or not.
I've yet to see any disappointment with my wax build-up during the winter. On the prime car, it doesn't see winter roads or bad weather; However on my daily driver, it has to endure the bad elements, so the build-up is even more important for this car. In the spring, I'll begin my new wax season by stripping off the old wax and re-starting by cleaning and polishing (once!), then establishing my wax routine for the rest of the season.