I use cork gaskets & always put them on DRY. Whatever type you use theres a couple of things, If it's a stamped steel valve cover, make sure it is absolutley clean & straight where the gasket rests, typically over the years these covers get over torqued & the rails are bent all to hell. If you get the rails straight, clean & you don't over-tighten the bolts, they should seal up for years & years with no trouble.
