It is not worth it to treat all customers the same because they are not. Some own the bulk of the potential market profit margin for your company. You want to concentrate on A and B customers and raise profit margins on C customers in order to justify the time used to satisfy them. Analyze their profit margins, location (close or far to service), time spent, difficulty to satisfy, potential to grow (more eligible company locations) and their influence on causing your company to grow and innovate. Companies that force you to innovate and develop new products are worth the time. Choose the criteria most important for your growth for evaluation.
CAUTION: The analysis is not the same for each company. Determine the ratings not on just their current purchases but their potential purchases. For example, one customer may be only buying $20,000 per year, but that customer has 5,300 sister divisions. Assign a sharp sales guy to that potential goldmine customer to take advantage of all the potential intercompany sales it owns.