I would leave the bottom in (closed) all year round ESPECIALLY during the heat of summer. Otherwise they can't cool the hive. Evaporative cooling does NOT work if you have wide open ventilation.
The hive I lost this winter had an opened SBB. (It wasnt a very strong hive so it's really not a valid 'test')
I had 7 hives going into winter with 6 being virtually the same strength. I covered 5 of the SBB's and left 2 open. Out of the 6 remaining hives on fairly warm days that we've had here lately, the hives that has closed SBB's are doiing house cleaning chores and flying around. The last one with an open SBB is still clustered up.
From now on, the SBB's will only be open for mine during the heat of the summer. No more open ones for me year round.