I recommend requesting a new W4 form indicating "exempt" from federal taxes, otherwise, if the IRS determines that an employee does not have enough withholding, you will be notified to increase the amount of withholding tax by issuing a "lock-in" letter that specifies the withholding arrangement permitted for the employee. Once a lock-in rate is effective, an employer cannot decrease withholding unless approved by the IRS, so request a W4 from employee before just not withholding their federal withholding.