Pit beat me to this one...

Your UTH is there solely to provide a hot spot which aids in digestion. Your ambient temps are controlled by an additional heat source (Heat bulb, CHE or RHP).

All heat sources should DEFINITELY be controlled by a thermostat.

...And like Pit said, a regulated heat mat will NOT melt plastic. Think about it like this: If you want a hot spot of about 92, then your mat will need to be set somewhere around 95-99 depending on ambient room temps, etc... Your body temp is around 98 and does not melt plastic, so your mat will not either.