Has there ever been a death where the main cause (or even a contributing factor) was cited as the canister being attached to the backplate with a metal to metal connection? There are alot of ways of dying while diving. I think your risk of death mounting a canister to the backplate is tiny. At least not any larger than many other things. I know the risks of metal to metal connections. I would be more concerned with metal to metal connections on stage or deco bottles before the canister.