I was taught to soak cork gaskets or even apply a generous layer of chassis lube so that the gasket wouldn't stick to the clean metal surfaces. That way you could pop the valve cover to adjust the valves without the cork gasket destroying itself.
Scraping a "stuck" cork gasket off the valve cover seating area could result in pieces falling into the internals. On the pan gasket, we used "permatex #2 to seat the gasket on the pan and greased the seating side with chassis lube to get the same effect. We didn't have a parts house on every other corner and it just made good sense to save the gasket. A broken gasket could be salvaged with a dab of #2 Permatex and you could get a new gasket for the next scheduled maintenance.
Dave
