DS: Do you ever tell people what they feed their dog is bad?

If someone mentions their dog has rashes, severely dry skin, is itchy etc I'll mention it may be a food allergy and tell them about how a lot of dogs are sensitive to cereals and corn. Other than that, never, if anyone tried to tell me my food was crap, I'd blast them. It's absolutely none of their business what I feed and unless I specifically ask them for a good brand of food, they have no right to say anything.