Horse apiece. If you're going to use decals I would suggest you apply them after you varnish. No telling what the varnish or poly will do to the material/plastic that the decal is made of. The decal can be removed at anytime using a heat source with caution.
If your transom name is painted on you can varnish over it since there are no plans of ever removing it in the near future. If you do decide to change a boat name you would be well advised to know the procedure and ritual lest you endanger your boating experience.
Modern vinyl decals are fairly durable without having to be protected. However, if you have one in a location where it could get abraded or scratched and you want to protect it, you can put a piece of clear plexiglass over it .