Billing Contacts

It would seem that billing contacts are no longer required to generate customer invoices in R12 as I have been able to generate revenue and invoices without them.   I did run into a problem for intercompany invoices though and had to add a billing contact to the customer and to my project to generate the invoice without error.  Does anyone know if this is the appropriate behavior?  I did not find any related R12 bugs on Metalink nor could I find any refereces in the user guides except for one reference in the implementation guide that said 'you can optionally define a billing contact for a contract project.