Hi Steve,

No, you can't have the "Ship-to Contact" value be blank and the "Bill-to Contact" value be non-blank, without using a ship-to address code. The way this process works in Adagio is that leaving a default value blank will want to refer back to a different place (ie the customer 'Header Contact' in this case) in order to read information. So that a different person other than the 'Main' contact for the customer could be set up for receiving invoices. Exactly as you outlined in your second paragraph.

Without a ship-to code, the Bill-to and Ship-to information will be identical on a new order. Though values can be edited.
With a ship-to code, you can default a blank Contact for just the ship-to section, which may also be undesirable for your client.

The choice is yours.