Colorado has Western rattlers, which get big and mean, but they don't live above about 6500 feet. Timber rattlers have been spotted as high as 10500 ft, but generally live below 9000, so I'm pretty much in the clear when out hiking.
It's actually against the law in CO to kill a rattlesnake that isn't directly threatening human life or property.
Fun facts about rattlers: they're one of the few reptile species that give birth to live young. And like all pit vipers, they will bite and inject venom when a warm object is placed in front of the pits, even if the snake is dead.