Marking your US Dollar bills
Is it legal?
(Image from StampStampede.org)
Yes, it is legal [opens StampStampede.org]
The law in the United States is written like this:
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. [source]
To be safe from repercussions of the law: Do not mutilate, cut, deface, disfigure, perforate, unite or cement together the bill with an intention to render the note unfit to be reissued.
You may draw small stick figures, stamp or write notes and messages on your US Dollar bills.