Recommended torque on the valve cover bolts in the M116/117 V8 engines with new crush washers is only 3 Nm - very little. Quite different from the diesels. Using old washers and a hardened old valve cover gasket probably increases chances of warping and/or leaking.
It's also very easy to have the gasket slip and not mate properly with the base of the cover (bitter experience speaking) so it makes sense to check the rubber seal all around with a bright flashlight before torquing down.
You can check valve cover warping by placing it on a really flat surface - a sheet of glass should work well.