The 25th anniversary of the voluntary motion picture rating system is being celebrated with a new opinion survey that gives it a 75% approval rating from parents.The ratings system, jointly conducted by the Motion Picture Assn. of America and the National Assn. of Theater Owners, has rated more than 11,400 pix since its inception on Nov. 1, 1968. It has enjoyed an almost steady climb in its acceptance since. The 75% approval rating has grown from 51% for all parents with children under 13 in 1969. The survey also showed that only 3% of parents with preteens had never heard of or used the system, down from 36% in 1969. The survey was conducted by Opinion Research Corp., Princeton, N.J. For parents with children under 18, the survey showed that 74% consider the system useful, up from 46% in 1969. MPAA prez Jack Valenti and NATO topper Bill Kartozian said the survey correctly reflects the scheme’s public acceptance.