Every time I walk into a location I look around and make note of who and what is where. I think about how "I would do it" if I were going to do whatever, and take a seat or sit at a table that is appropriate to that particular scenario.
It varies based on what I am doing and where I am.
But generally it is never in the "center" of anything as the human eye is always drawn to the "center" of anything and being "off the X" to begin with is usally an advantage.
Unfortunately sometimes the "center" will change based on the offenders intent. Middle of a large group of people vs cash register vs direct line for quick exit vs the middle window in a group of 5 windows and so on.
I remain alert without being paranoid and I try to enjoy myself. I keep myself alert by pointing out things to see if my wife is paying attention. It is great fun sometimes. Life won't be worth living running around in Condition Orange/Red ALL the time. You will probably die of a heart attack or stroke before you ever get shot from all the stress.