If all mating surfaces are undamaged and clean then normally I don't use gasket sealer.If I'm re-using and old gasket which I have had to do from time to time then yes I use GS but applied correctly e.g with a small paint brush,thin layer,gasket pressed and then ALL excess sealer removed/wiped clean especially engine side.The less debris in the engine the better.
I have used GS on heads when testing different engine builds to try and preserve the Head gaskets to be re-used but under normal circumstances the Dry gasket is usual but the Manuals usually say to use some.
I'm led to believe that Trials riders use Copper Grease as a gasket sealer so they can re-use their head gaskets as they tend to remove their heads a lot and the cost of new gaskets ll the time would be prohibitive.
There is no harm using GS if applied and cleaned off correctly and will ensure a good seal.
HTH
