Not anywhere I've ever shopped.
The closest I've ever seen is the Walmart greeters, on the rare occasions when I can't avoid going to Walmart.
But then, I live in a rural area, and almost exclusively shop in suburban and small-city stores (the largest city I regularly shop in is Syracuse, NY).
I could probably count on one hand the number of times I've seen someone who looks like they were placed to discourage armed robberies. Although I have frequented a few grocery stores that had police hanging out.