Once you have sealed your cake with the crumbcoat layer of icing then you have sealed in the moisture as well. So no worries about it getting dried out. Once you have it crumbcoated wait at least 3 hours for it to settle...overnight is even better. This not only allows the filling to sploosh out and you time to smooth it again, but also allows air and gas to escape as the cake settles and comes to room temperature. You don't want to put on a final coating of icing or fondant on a cold cake or before the cake has had proper time to settle or you may end up with air/gas bubbles giving your buttercream a bursting air bubble (a blow out) or your fondant an air bubble. So go ahead and fill and crumbcoat your cake and know that it won't dry out on your counter after this step. I would always advise to serve the cake within 3 days to assure freshness but have had cake 5 days after the final coating and it still be wonderful.