I do most of my own work, but I occassionally have an LBS service my bike. I think that it's nice when the shop retains any replaced parts, and shows them to me with a brief description of why they needed to be replaced. It really doesn't take that much time, and it allows the customer to see where their money went, with their own eyes.

Not every customer is interested in seeing the replaced parts, but simply offering the little show and tell session conveys a positive message to the customer.

My girlfriend doesn't know anything about automotive mechanics, but she always appreciates it when her mechanic shows her the problem. It is the complete opposite of the male chauvanist attitude of "your little female brain wouldn't understand these greasy parts."