Sometimes that's the case and sometimes it's not. Like in The Princess Diaries, which was shot in open matte, there ended up being an unintended boom mic in frame on the fullscreen version. But Zemeckis has been quoted as saying that he shot it in 1.33:1 because he knew that more people would end up seeing it on a home format anyway. And Bob Gale said on the commentary that they didn't have any interest shooting it in widescreen because they didn't consider Back to the Future to be a movie suited for any wide angles. It was just matted that way because it had to be so for theatres.