We do not encourage the defacement of U.S. Currency.
If you choose to mark your bills, please do so in a reasonable and prudent manner. Basically the bill must not be "unfit to be re-issued."
If you deface the bills in such a manner that renders the bills "unfit to be re-issued," you may violate the law.
Click Here to go to the US Department of the Treasury site concerning marking on currency.