I started with SBB and have usually left them open. I put slid the cards in this past winter to see if it would help the survival rate. One made it, one didn't, so no conclusion. I have left the card slid in so far this year because I'm too cheap to buy a solid bottom board. But if they get to bearding, I may slide the card out. I'm still experimenting with the bees to see what works and what doesn't. (On second thought, that experimentation never really ends, does it?)