I have lived in southern Arizona for nearly 14 years, worked our local golf coourses as a marshal for 5 where encountering them, especially in the spring, was nearly a daily event {often saw half a dozen and had to move 2 or 3 a shift}. To all of you folks who have spent time in the southwest but never have seen a rattlesnake I suggest you pay a LOT more attention.
Their natural camoflage is off the charts effective. I have been in the process of carefully relocating one on the golf course and glanced away for a second. When I looked back it had virtually disappeared... but was still just a few feet away. No they do not always rattle and do not have to be coiled to strike/bite. Those myths can get you bit very easily.
If someone is struck get them directly to medical facilities ASAP. No cowboy nonsense {don't make cuts and try to suck out the venom, apply ice or a tournequet - just transport them ASAP}.
The early spring, following the first week or so of 80 degrees+ is the most dangerous time of year. They have been denned up all winter and are hungry and horney actively seeking a meal and a mate though not necessarily in that order. The point being if you see one I guarantee you there is another nearby and it's the one you don't see is going to be a problem.
As noted, please don't kill them unless they pose an immediate threat to your family or pets.
:C