I just replaced my HG this past weekend. It had some imperfections on both the block and head. nothing major, just tiny bits of pitty and scratches.
I used copper coat on both sides of the head gasket...
Started right up and ran better than ever and i've already put 200 miles on it with no issue.
Moral of the story: if you have 2 perfect surfaces, dont bother using it. If you need to fill (minor) gaps in the surface, use a bit. If you've got major damage and you're cheap, slather it in copper coat but dont ever expect to get them apart again lol