Hi Mike,
I did a quick test based on your question, and I do see your point, I found in QuickBooks desktop, you can only add Payment Terms for a Customer, the option simply isn't there for a Job. I then checked out Method, and again agree with you, you appear to be able to choose the Payment Terms for Customers, as well as Jobs.
I will have to do some more digging however to confirm what would be the correct way to do this from Method. I have a feeling, that we would need to mimic how QuickBooks handles this on their end in order to avoid conflicts. I will post to this thread as soon as I have some specific answers to your question.
-Ben