I don't have the book in front of me, but I believe J.W. Rinzler's The Making of Star Wars says the rating was the studio's decision (Fox). It was them, not Lucas, that ensured the PG-rating from the MPAA. Anyone who has the book care to check?
That's right, Alan Ladd Jr. and the studio took the lead on that, not surprising since it was more marketing than a filmmaking issue. The story in the book is that the MPAA issued a G in a close vote. The studio requested a reconsideration for a PG. The Star Wars
publicist, Charles Lippincott, saw a young boy at the preview screening start to cry when Vader choked the ship captain and felt that a PG was really warranted. Lippincott knew a woman who was on the MPAA rating board, and she told him that she and another woman had wanted a PG but some of the male members hadn't really paid attention to the movie or had fallen asleep. After the review, the PG was granted.