Really it is. It is really necessary every time an employee comes in to work he must have a very long patience. Because that very long patience can be consumed by ONLY ONE psychotic customer.You need to suck it most time for the business. It doesn't mean that you can take any disrespect that a customer shoves down your throat. However, your reaction and attitude towards the way you handle the situation matter a lot. Be polite and calm while pointing out the problem.