I used nothing but rheostats (dimmer switches) for many years before ever buying my first thermostat. Frankly thermostats weren't even an option because they didn't make them in a small plug-in variety with a remote temperature probe. The thermostats that you can now easily buy on amazon or at reptile basics simply didn't exist. And you know what? Rheostats worked just fine as long as you monitored the temperatures in other ways (IE: with a thermometer) . Rheostats don't adjust the temperature they just keep the power at a steady level.
A thermostat will keep your heating element at a specific temperature (say 90 degrees) so if the ambient temps in your room rise to 85 degrees or drop to 60 degrees your heating element should always stay at 90 degrees. A rheostat on the other hand will just limit the amount of power to the heating element and pays no attention to temperature. If you have the rheostat set so that your heat tape is at 90 degrees in a 70 degree room and your room temps drop to 60 degrees your heat tape temperature will be 80 degrees, if the room temp rises to 85 degrees your heat tape temperatures will be at 105 degrees.
So, as long as your room temperatures are stable dimmer switches work just fine, if your room is prone to wide temperature swings you should consider a thermostat instead.
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