I have noticed loss prevention being fairly pervasive, but I have rarely noticed actual security guards. The difference being loss prevention is there to stop shoplifting, and are generally instructed to never use any physical force. They'll take a description and let the shoplifter get away if the situation becomes violent.

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.