I have never locked my checked bags. There's nothing in there of much worth. I did have one necklace stolen many years ago. Flew out of Boston into San Fran. It was a really good heavy costume jewelry necklace...looked like it cost much more than it really did. I'm sure someone thought they were getting a very expensive piece of jewelry....well, at least until they tried to dump it.
Nope, have no reason to lock my bags.