From United States Code: Title 18 Part 1 Chapter 17, Section 333:
Mutilation of national bank obligations: 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 any 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.
There is some online discussion about it. But it should be legal.
http://www.stampstampede.org/pages/stamping-tips Stickers on coins are legal fo'show.