My experience with copper gaskets has been that they will weep a tiny bit making a mess on the heads below some of the ports. I have a little more water content in my exhaust during cold startup than most. Since I've started using copper silicone sealer (any high temp silicone sealer) the mess has disappeared. The leaking was never bad enough to call it an exhaust leak, it was just causing an annoying mess. On a black colored head/block you probably can't even see it. Mine is unpainted aluminum, so it really shows up. It only requires a very thin coating. You don't have to get carried away with it so that you end up with an ugly mess around each flange. I use a thin coating on all my exhaust gaskets. After many teardowns and reassembling, I can always count on the exhaust gaskets sealing. The sealer has never let me down.
Again, only a very thin coating is needed.