I've always used sealer from Forcelle and Clark with good results. The copper gasket surface(both sides) and deck surface must be really clean to get the best results. If you were using a steel O-ring wire I would be more concerned about the varying ring height. With copper wire, it will squash flat when the head is torqued and won't even leave an impression in the gasket like steel wire will. You will have no additional sealing benefit around the cylinders with copper wire because it is just too soft the perform the job it is intended to for your satisfaction.
