From my perspective, customers should be viewed as having great value. This is an entirely different thing from saying that the customer is always right (which by the way contains the additional team-killing undercurrent of suggestion that management will always back the customer over their own employees).

Some customers will use vast amounts of your time and constantly expect you to accommodate them. Especially in a small operation, the customers that cause disproportionate expense are toxic. These are the ones that you need to actively fire, and the assumption that every customer is a good customer (or that every customer is right) quite simply precludes this option.