Your much more likely to damage either part using a scraper as oppose to abrasive paper. If you have to take the head to an engine specialist because you cant remove the old gasket there is something wrong, either its melted on and the head has been extremely hot or your trying to remove it with a flump.
I think we will have to agree to differ on that one. If you scrape correctly you do not remove material and if you use an abrasive, you cannot
help but remove material. The OP was talking about sealer residue and carbon, not melted gasket, which should scrape off fine.
Heads are faced to extremely tight tolerances of flatness and finish and 'sanding' is going to ruin that. A wooden block might look flat to you, but it is not a machined tolerance, just guesswork.
Yes, I am qualified in this subject and do know how hard it is to hand finish something flat. The answer is very hard, like you need proper apprentice training and a lot of skill.
It may be that you can get away with doing this and not have problems, but as I said, I would not do it myself.