What, like a good idea like you would always do it or good idea you should probably do, but dont always do it.
A luthier told me to dont that to finish over the decal if you mix a little white glue into the water you soak the decal in. He says they harden up pretty good with that method and are durable enough like that. I remembered this little perl of wisdom after I had finished applying it.
I can finish over it no problem, but I have to get my luthier to do it as I cant get the right lacquer I need to do the job myself. Satin non uv protect lacquer is impossible to find here. My luthier is very good, but he takes an age to do any sort of finish work and I know he is busy with a lot of jobs at the moment.
It's purely subjective. For longevity's sake (if I were to apply a waterslide decal) - *I* would probably apply a finish over it. That's just me. One has to consider the quality of the decal, as well. Some are good - some are not (and consequently smear/run). As it is......I have oiled Warmoth necks, and have no decals or identification on the headstock. YOU have to make the call...