Overnight retard for soudough

Hi everyone, 

When I retard my sourdough overnight in the fridge, I cover my dough in the proofing baskets with plastic shower caps. However, I recently saw a video on YouTube by Bake With Jack, and he suggests leaving them uncovered in the fridge to form a skin. 

Do you retard covered or uncovered? Would there be advantage to forming a skin on the outside of the dough before baking?

Thanks.