Dear John,
Q. Why shld I deselect Payment Per Due Date?
Ans. You you shouldn't use it, if your customer don't want to sort or groups their open invoices for payments.
or
they don't want to separated according to payment method supplements and don't want to be printed separately by these supplements characteristics (configured and assigned in master data or in documents)or sorted by them for printing.
I can understand your worry, but as you know this is not system issue, its a belongs to customer needs, so I would advise please discuss this with your customer, either they want to use the Payment Method Suppliment or not, and act accordingly.
Regards
Javed