I dunno, personally I understand with an Under Tank heater why you would put it directly on the heat source because you don't want your snake to get burned, but with a ceramic heat emitter and a thermostat, I don't want the emitter to be 85 degrees, I want his TANK to be 85 degrees. And I feel like trying to estimate by saying okay the heat emitter can be 90 degrees because when the emitter is 90 the tank is 85 is just kind of a waste of time and energy trying to figure out where the perfect range is. I put my probe inside the tank below where my emitter is so that I can set the temperature of where my snake lives.
I don't know, that's my personal preference. If I had a direct contact heater like an under tank heater, I would DEFINITELY put the probe directly on that thing to make sure my snake doesn't get burned.