Secondly, you need your own relationship with the good customers so as to provide a backup if the salesperson leaves. In the worst-case scenario, if a disgruntled salesperson leaves and joins the competition, you need to know who the customers are, and they need to know that you are the face of the company behind the front-line salesperson.

Notice that the emphasis here is on “good” customers. I don’t think that you need to know every customer, nor do you need to know the prospects.