Hmm, to answer your question, I would say probably never. "Before the film begins all the tickets are being collected" sounds really bad. Switching the order ("tickets are being collected before the film begins") still sounds bad, though not as bad. If you are explaining how a process will work, you might say: "Before the film begins, all of the tickets will be collected" "We will collect all of the tickets before the film begins." Using future perfect is OK, but sounds a little more formal than necessary.