I have never once needed any RTV on my YJ cooling system. All the factory ever did was use paper gaskets, clean surfaces, and proper bolt torque. that is the same theory I use and it works. I use Mopar gaskets. Be very careful that the thermostat doesn't slip out of the recess in the cylinder head. If it does, it will get clamped down between the housing the the cylinder head and that will be impossible to seal. Get the surfaces fully clean before trying to mate anything together.
I know the front cylinder head bolt needs that on both engines but I’ve never heard of that for either 2.5 or 4.0 thermostats. The 4.0 certainly doesn’t need that, mine don’t protrude into the water jacket.
I have the same complaint (issue) with the vertically mounted thermostat slipping out of its cylinder head recess while trying to assemble the gasket/cap. Not the best design.
Kind of a pain for sure, although it typically doesn’t give me too much grief. It can be more of a pain with a tummy tuck and no motor mount lift since that makes the front face of the engine more vertical. But overall I don’t have too much grief. Could be better though.
