I use a Tone Gard on a varnished instrument. I can see light marks where the Tone Gard arms have been. Still I don't worry about it, as whatever little scuffs I may get on the sides, I'll save the back from all of the scuffs it would get if the Tone Gard weren't there. Plus it makes the instrument sound better, and I'll gladly trade a few scuffs for a better tone. Just my $0.02.
"Aerodynamically the bumblebee shouldn’t be able to fly, but the bumblebee doesn’t know that so it goes on flying anyway."
Newell A5 #37, Glenn F5 #66, Eastman 615 #537,