If your Snake rests on top of the probe it could feed back incorrect temps to the t-stat I believe.
Also your snake could go to the bathroom over the probe damaging it, causing it to give incorrect temps back to the t-stat.
Having it on the outside would give me peace of mind. I had done this for 3 years and never had a single problem.
When I put thermometer probes into my snakes warm side hide, if he lay on top of them it would always tell me the temp is much higher than what it was.