Valve cover gasket: cork or rubber?

hi-tack or rtv on the valve cover, then install the gasket into the valve cover. once assembled and cured you can R&R the cover alot of times, im up to 10 times on my 68 without having to replace the gasket, and no leaks.

that being said i have used the rubber ones, and could not been able to get it to seal. Would not seal even with rtv on both sides of the gasket, sand blasting the sealing area in the valve cover, and scotch briting the head surface.