I'm pretty sure ours aren't allowed to accept tips. I know at my favorite store, the guys who walk the groceries out to your car and load them for you are not allowed to accept them.
I consider the tip is the fact that I'm even buying the groceries with their ridiculous markup.
That aside, I don't understand the overtipping culture. If this is a service they offer, and they pay their employees to carry out the service, why tip? If you tip them, why aren't you tipping the person at the checkout? Or the produce stackers? It makes no sense.
I think people in NYC tip every fucking person ever. It's annoying.