if you’re going to refrigerate overnight, then I would suggest putting your completely iced cake into a cake box first and also be sure you don’t have any odor-transferring items in your fridge (i.e., you don’t want your cake smelling like an onion the next morning). I do this all the time and haven’t had an issue with drying. BTW – you could also cover your cake in fondant the day before, pop it in the fridge (in a cake box) and continue working on it the next day . . . just be sure to leave your cake alone (don’t touch it) for an hour or so when you take it out of the fridge . . . let it come to room temp because it will sweat, but if you leave it alone, the condensation will disappear. When doing this, I’ve found that the fondant will work as tupperware and ensures the cake doesn’t dry out.