Is sealant really necessary on a thermostat gasket?

If the surfaces are flat, I have good success dry. More so than warped and RTV.But if the housing with the recess has been straightened one too many a time, such that the recess is no longer deep enough, nothing will seal it. The recess would need to be re-machined( die-grinder). And those late style, thin metal housings, can be a PITA. The coolant likes to work its way over to the bolts and come up the threads. So that would be a good place to put the RTV; on the threads and under the bolt-head.Do your best to prove that housing is flat, especially the stamped steel one.