In everything I have read they put 1/4 hardware on the bottom to keep the rodents out.
But those who use the hardware cloth are not using slatted racks. A slatted rack puts over an inch of distance from the hive body and the stand, just like a SBB would. It also puts bars on the window to keep skunks, etc, from getting into the hive from below. I have not found any noticable downsides to bottomless hives the way I am currently doing them.