The local Walmart (which I just visited today) hasn't had regular "greeters" for quite some time. However, they have a "checker" to keep an eye on what's going OUT OF the store. That person may ask to see your sales receipt and match it against what you've got.
They almost never bother me for some reason.
I've also come to use the "self-checkout" stations all the time now. I've caught cashiers making mistakes before. I'll make my own mistakes, thank you very much (and boy, do I make 'em!).