It's also illegal to "deface" currency...if I could, however I would put something like "go to hell" with an arrow pointing at this stamp
Technically it is. I just said a lot of people do it, and why. I don't think the likelihood of getting in trouble is very high. When I was a cashier I used to have customers flip out on me every once in a while when I would use a counterfeit testing pen on their 50s and 100s because it would leave a pale yellow mark on real money (and a black mark on fake). When they saw me test the big bills with the marker they would say "You can't do that! It's illegal!" I told them counterfeiting is a much more serious crime and nobody had tried to arrest any cashiers for the markers yet.
Then they would either get a pouty face and get quiet or they would pout and squeal "but it's ILLLEGAL!"