That can't be true. VI was PG. At worst it was getting PG 13 rating and the lack of vulger language wasn't going to allow that to happen. If any TOS movie deserved above a PG rating it was IV. And it was a PG movie.
Here's a good FAQ answer. The first ST movie to get above a PG rating was First Contact. Nemesis also got a PG 13 rating. All the other Trek movies were PG according to IMDB.