(1) The parties to any proceeding or investigation before the commission may, by stipulation in writing filed with the commission or entered in the record, agree upon the facts or any portion thereof involved in the controversy, which stipulation shall be binding upon the parties thereto and may be regarded and used by the commission as evidence at the hearing. It is desirable that the facts be thus agreed upon wherever practical. The commission may, however, require proof by evidence of the facts stipulated to, notwithstanding the stipulation of the parties.