Once an employer has become liable to a return of withholding, he must continue to file a return even though no tax has been withheld, until such time as he notifies the Tax Commissioner, in writing, that he no longer has employees or that he is no longer liable for such returns. If an employer requests in writing that he be permitted to change from a monthly return to a quarterly return on the ground that his withholding has become less than $300 for each quarter, such change shall be permitted only at the beginning of a calendar year.