Every time I've seen a rattler in the wild I didn't really think in safe distance terms so much as getting as far away as fast as possible. I spotted a juvenile rattler in the hills of Riverside when I was hiking with my brother in law, foolishly mentioned this, and the crazy bastard started poking it with his fishing pole. This further scared me in a situation where there was already a rattler nearby.
Also, as far as I'm concerned there's no safe distance from cougars. Never seen one in the wild, never want to see one in the wild.
Humans, though, no way to keep a safe distance from all of them, so you just have to live wth the fear. It's worst when I have to get haircuts and do small talk, but I quit cutting my hair, so that's less of an issue.