Ermmm both.
I usually have a hot shower / bath beforehand anyway (night shaver) then use a couple of hot flannels while my brush is soaking. When I actually start shaving though, I switch to cold for rinsing the razor & washing off between passes. That's mostly because I can feel my wallet contracting if I leave the hot tap to run straight down the plughole!
I've been carving up my chin something terrible lately, but I'm certain it's down to cack-handedness rather than water temp.