RTV, Gasket Dressings, Gasket Maker in combination with a flat gasket, etc. All have their advantages and disadvantages, we can argue all day whether its better to use them instead or with a gasket.
EXCEPT a HEADGASKET. That is the most stressed and critical gasket of the whole motor. Not to mention, the most expensive and difficult job to replace if you screw up the job.
Now years ago, when headgaskets were all flat metal spring steel gaskets, Copper Spray and some other sealants developed help improve the seal etc. But over the years, they have drastically changed the Headgasket designs, using all sorts of different exotic materials intended for all sorts of different factors in sealing the head to the block. A lot of the new Multi-Layer Steel (MLS) Headgaskets are coated with a Neoprene Rubber Layer to act as a fill for surface imperfections in a far better way than any spray or squeeze on sealant will, the extra sealant would only defeat the purpose of that coating and most likely make it more likely the gasket will leak.
When I did the HG on my little Neon R/T, the instructions specifically recommended a special spray on Sealant by Part Number. I used what was recommended and it was different than any spray on sealant material that I've ever used, but it worked great and was specifically designed for my MLS Headgasket.
Although the Copper Spray Sealant is great in hi-temp environments with lots of pressure on the flanges (I use it on Exhaust Gaskets all the time) I wouldn't use it, UNLESS YOU KNOW IT IS COMPATIBLE AND WORKS WITH THE HEAD GASKET AND NOT AGAINST IT.
Unless you know better, from real knowledge, NOT just guess on your part; I'd follow the manufacturer's instruction to the letter. Use only the sealants they recommend, if they recommend NOT to use any sealants, than follow those instructions.
You don't know better than the guys that spent a lot of time and engineering to come up with that headgasket. Its just to tough of a job to re-do because you think you have a better idea.