The previous advice is mostly correct, namely dont give any indication that you disagree with this philosphy. However a more insighful answer would be: "Yes, the customer is always right, but sometimes it may not be my place to tell them so." This reinforces the belief in the customer but also reinforces the belief in the company and its rules. For example, if a customer returns something that may or may not have come from the store and does not have a receipt- allow the manager to make the final decision.