The reasons to use gaskets is not related to temperature. I don't see any reason why it would change with or without gaskets.
Most people are running old engines with imperfect manifolds, so should use gaskets. If I was building and planning on not using gaskets, when I had my heads surfaced, I would ask to have the exhaust side surface too. Then of course you would have to have the manifolds surfaced also and with careful torquing, you should be good to go.