Last month I flew from Detroit to Greensoboro, North Carolina, and back with my Martin D-28 checked in its original (1962) case, strings slackened, neck and sides packed in bubble wrap. I locked the latch with my little tiny key. Nobody told me I couldn't. Guitar and case came through unscathed.
However, a friend of mine who flew from Greensboro to (get ready) Providence had her relatively new Martin guitar case badly dinged and gouged. The guitar was fine.