I say no heat pad on the carpet. Fabrics and soft surfaces hold on to a lot more heat than solid surfaces, and don't reflect it back the same way that say wood or tile does. This can cause the heating elements to malfunction because of the built up heat, which obviously, is bad whether your heat pad stops working completely or starts a fire. All heating pads (human, reptile, dog, etc) are designed to be used with one side exposed so that excess heat can dissipate into the air, with hard surfaces you can usually get away with placing the pad right under something as light weight as a roach bin because the solid surface just throws the heat back up into the bin, but on carpets, you're practically asking for a fire.
I use a tile between my heat pad and the floor, and it is propped so that there is some air space for heat dissipation. My heat mat is also on a thermostat, so that helps.