You are probably scratching your head now, thinking "wait, what? did he really mean trustworthy customer?"
Yes. Yes, I did. You and I were raised with "the customer is always right" and other slogans that make it clear that a customer can be whatever they want to be, and it's up to you, the salesperson, to close the deal. The customer has zero responsibility. Definitely not to be trustworthy...
Well, consider the following scenario. You got a purchase order from a customer. You performed the service (or delivered the product). You sent an invoice for the agreed price and payment terms. The due date arrived, and yet there was no payment. You gave it another day or two, or even a week. Then you sent an email. No response. So the next day you called. You started getting the runaround between who's responsible for payment, and you heard stories about the checks not being cut for the month yet, or that they were cut, but they are only being mailed once a month, and you just missed this month's mail date, or whatever.
A few months later, the same customer issues another purchase order for yet another product (or service). You wouldn't decline the business, right? But, at the same time, wouldn't you increase the price, reduce the discount, shorten the payment terms, or do something to compensate for what you know will follow? Maybe this time you will not perform the service or deliver the product before you were paid in full?
Most customers don't do that. At least most of my customers don't. But if someone does, they will be paying for that the next time they order something.
Now, consider the opposite. The check arrived three days before the due date. On the due date, the accounts payable manager at the client's company called you (unprovoked...) to make sure that you have received the check, and that everything is to your satisfaction. What do you feel about this customer? What would you do if this customer asked for a discount or better terms next time around? Wouldn't you be more likely to agree?
So, be a trustworthy customer. It actually saves you money.