After using Hasselblad cameras for a few years, I've always left the dark slides out until needed. Cameras with backs with film in or not all have no dark slides installed. Correct me if my thinking is wrong, but leaving the dark slide in compresses the foam seal and when left in all the time eventually the seal stays compressed when the dark slide is removed and voila, light leaks. Even film backs w/o film not on camera have no dark slides. I haven't ever had to replace the seals on any of my film backs but maybe I'm due!
Just thought sharing my experiences might help.