Did a search and found the latest discussion was late 2002, Lost of discussion, opinion, positions, but little "fact"

Traveling LAX to MEL and then within Australia. One of the bags is a Samsonite hard side that has built-in locks (dated) with a simple key. None of the rest of our luggage locks. We normally lock this bag as much to keep it from "poping open" as for security.

Are there rules to lock or not to lock your checked luggage given these facts?