Even the little pre-made cartons of pudding need refrigeration, once they're opened.
I refrigerate my fondant-covered cakes and have no problems. I use MMF and live in a very humid climate, but as long as you run your air conditioner, the inside humidity and the outside humidity don't have much to do with each other.
For the OP, because this thread has gotten way off of the original question, you can refrigerate your cakes, or if they don't have any fillings that will spoil, you can leave them out on the counter. The fondant helps to seal in the moisture, so a fondant-covered cake that hasn't been cut into will be much moister at 3 days on the counter than a cake frosted in just buttercream (or crisco-creme for you picky word people