Except for the part that its illegal to purposely damage currency (USD anyway) I don't think its very enforced, unless you are melting down pennies or something like that.
"Whoever mutilates, cuts, defaces, disfigures, or perforates, or unites or cements together, or does any other thing to any bank bill, draft, note, or other evidence of debt issued by any national banking association, or Federal Reserve bank, or the Federal Reserve System, with intent to render such bank bill, draft, note, or other evidence of debt unfit to be reissued, shall be fined under this title or imprisoned not more than six months, or both."
Yes I do see the "Intent to render such bank bill, draft, note, or other evidence of debt unfit to be reissued" but I'm sure if caught somehow, at the very least it would be annoying to have to explain to the police what you were doing.
18 USC 475 also prohibits writing advertisements or notices on US Federal Reserve Notes.
"Whoever designs, engraves, prints, makes, or executes, or utters, issues, distributes, circulates, or uses any business or professional card, notice, placard, circular, handbill, or advertisement in the likeness or similitude of any obligation or security of the United States issued under or authorized by any Act of Congress or writes, prints, or otherwise impresses upon or attaches to any such instrument, obligation, or security, or any coin of the United States, any business or professional card, notice, or advertisement, or any notice or advertisement whatever, shall be fined under this title. Nothing in this section applies to evidence of postage payment approved by the United States Postal Service."
The Secret Service got pissy at Where's George over selling rubber stamps that let you stamp wheresgeorge.com on bills.
So, if anything, they wouldn't care unless somebody started selling tools to mark the currency and promote Bitcoin.
But other than that, they generally leave Where's George alone 'cuz it's not a high priority.
