Food? The local supermarket might not be awe inspiring for food pictures and the staff there might get suspicious of people taking photos of the frozen pea's or whatever. Never tried it myself. I've never had a problem at public markets. I think if you don't get in the way of paying customers they don't mind. Probably like better if you bought some of your subjects after. Sometimes the way the food is arranged can be rather pleasing. It's all how you look at it I guess.
Now that I have said food pictures in a supermarket might not be awe inspiring, I wonder if I should wander in there with my camera.