Those using screened bottom boards, do you close them up for the winter or has anyone tried leaving them open? I'm sure this varies where you live, but some beekeepers are saying that they think it's best to leave them open for increased winter ventalation. They say that the bees can maintain their cluster temperature regardless of the big hole in the bottom and it doesn't hurt them. Has anyone else tried leaving them open, and what are the benefits and disadvantages? Thanks!