The probe should be placed in the hottest possible area your snake could ever reach. So, on the bottom glass (assuming you have a UTH on an aquarium) would be the best place for it. Don't worry...if a snake gets too cold he will move to a more comfortable spot.
Edit: I just checked mine with an infrared temp gun...the bottom of the tub is 84.7* and on top of an inch of aspen the temp is 79.8*. That's acceptable for me...