It was a rule long before then, at least by the late '60s when I first started learning. Can't remember when Cooper codified his rules, but I'd suspect it was

It was a rule long before then, at least by the late '60s when I first started learning. Can't remember when Cooper codified his rules, but I'd suspect it was earlier than the '80s. However I was taught by my father in the hunting domain, where Rule 2 rules, "Never let the muzzle cover something you're not willing to destroy", i.e. keep it pointed in a safe direction. That was because you never knew if a twig or the like might pull the trigger (see the problem with holstering striker fired handguns which catch e.g. a windbreaker cinch tie), and you don't trust safeties. The iffy history of the Remington 700 underlines the importance of Rule 2 (known to fire upon manipulating the safety).