At the grocery store I shop at (a bit more "upscale" and thus more pricey, but totally worth it!) the carts have a back that lifts up and lies flat, so the cashier can just reach into the cart and pull items out, without having to lift them as well. I can only imagine how it saves on shoulder/back pains! I've noticed too that the counters the cashiers stand at are lower than other grocery stores, about waist height for me instead of rib height or taller, so it's not a problem to lift up items from a small basket.
Everywhere else, I unload my basket/cart as long as there is room to do so. I figure I'm the one who wants the heavy gallons of milk, or the laundry detergent, so I need to be the one who takes care of putting them where they need to be. This was solidified (in my mind) after I started working with disability claims, and saw how many people who work in grocery stores go out of need due to needs shoulder repairs, back surgeries, etc. It's just not healthy to have to lift that much weight every day, especially with some of the awkward positioning required by the cash register.