We only have experience with 1 site with DT Register so far.
As far as I know, these fields won't fill automatically when someone uses the registration form. Our clients have to go to the profile and fill the fields by hand. But after that, these fields are used to fill the reservation form. This only applies to the person who created an account. So: the billing contact.
So, I think this is a one way connection from profile > reservation form.