I keep all of my Ranco probes outside of the enclosures directly on the heat mat (or Flexwatt) because you never know what the snake is going to do... some of mine have a real talent for absolutely destroying their enclosures. If they end up moving the thermostat probe in the process, you might cook them under certain circumstances.

Keep in mind though if your probe is on the heat mat, the thermostat temp might not be exactly what you want the hide temp to be. For example, your heat mat might read that 110 degrees, but through the aspen and all of that fun stuff your hide might only be 80. That's where the backup thermometer in the cage comes in handy.