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If your career made headlines like all the ones on this simple Google search,
Police Officers ambushed
you would exercise caution as well. There is a difference between caution and paranoia.
That said it is always a good plan to not have your back to entrances etc. Can you always do it? No but its safe practice. One of the reasons you do try to do this all the time is to build muscle and mental memory. If you go to a good place to eat but it's in a sketchy part of town, even if only every now and then, you want good habits to help protect you. It's no different than repeating the same punch or kick over and over again. If the muscle memory isn't paranoia, neither is the mental, so long as it is practiced properly, simply as a matter of habit and caution vs. "omg mass shooters are everywhere."