I thought I read somewhere, perhaps in regard to professional liability (Circular 230), that once we received the signed permission, 8879, we had to e-file regardless of receiving payment.

Not in Circular 230.  And imagine the scenario, client sitting at your desk, you print the return and give the 8879s to him, he signs and says "AHA!  File my return now and I'm not paying you a penny!"  And not because of disappearing pennies.  

It does address having to return original copies of client paperwork regardless if a dispute over fees.