If, at any time after the petition is filed, the petitioner desires to withdraw the same, he shall file with the hearing clerk (or, if filed during the course of a hearing, with the judge) a written request for permission to withdraw. The judge may, in his discretion, thereupon dismiss the petition without further procedure: Provided, That, if the request to withdraw is filed after a hearing has been opened, permission to withdraw shall be granted only in exceptional circumstances.