I can't imagine that using a dremel to allow an 11-speed cassette to fit on a 10-speed hub would cause much of a problem, and the same would be true for any other method that would cause the material to be removed less than perfectly. The edges of the cassette's cog carrier that interface with the splines on the freehub body are the important part where the pedaling loads are transferred from the chain to the hub, and you wouldn't be changing those. The only thing that removing an imperfect amount of material would effect is how well the cassette lockring holds the cassette on, so as long as you tighten that well and check it once in a while then I can't see what problems you would have.
It would certainly be nothing like not using a jig to build a frame, and I expect that there would be no observable or functional difference between this "inferior" method and the "proper" method. There are certainly some things that need to be done precisely and with the correct tool, but I'm not sure that this is one of them.