i always used cork......just a little silicone to the valve cover and stick the gasket to it....dont use any silicone to stick the gasket to the head......then you can use the gasket several times before it leaks....you know like when you need to adjust your roller rockers.:yes:
I use the extra-thick cork and RTV them to the valve covers. The benefit of using the extra thick style is that the flanges of the valve covers won't touch on an aftermarket intake manifold impeding the seal. I can use the same set for years even with frequent removal to check valve lash.