An old hotrodder told me a long time ago that if you just kinda snug the bolts up a little while cold, then run the engine up to operating temp (or even a little hotter with the help of a piece of cardboard in front of the radiator), shut it down and tighten up the bolts-torque spec or whatever way you choose-as the engine cools it will suck the valve cover down even tighter. Once it's completely cool, re-torque the bolts again and it will never leak again. I don't know if it helps or not, but it's what I do and never seem to have any issues with leaky covers.
