I also make sure I return my cart. I dislike seeing them where they could get hit when someone pulls in. I also have a compulsion to make sure the carts are pushed into the next carts so they are in a straight line. My quilting eye needs to see the straight line I guess! LOL
I always let someone go ahead of me in a checkout line when they have only a few items. I am not in that much of a hurry that I can't wait a few minutes. I usually have a full cart so it makes sense to me not to make them wait through my long list of groceries.