26 U.S.C. § 7504: Fractional parts of a dollar

The Secretary may by regulations provide that in the allowance of any amount as a credit or refund, or in the collection of any amount as a deficiency or underpayment, of any tax imposed by this title, a fractional part of a dollar shall be disregarded, unless it amounts to 50 cents or more, in which case it shall be increased to 1 dollar.