i have read several posts about replacing the thermostat and none mention using any type of sealant, like liquid gasket. Is the rubber one that come with the new thermostat enough?

 

Yes. The rubber seal is all you need.

 

When you have a rubber seal and you use anything between the metal surfaces of the housing you are actually taking away some of the clamping force that provides the seal.
And if you have a chance now would be a good time to get replacement bolts with a hex head at the hardware store and dab a little "anti-seize" on the threads for next time