Originators need to obtain authorizations for both credit and debit entries. However, the debit authorizations need to be in writing with a signature or similarly authenticated(except for the RCK, TEL and ARC entries) whereas credit authorizations can be provided in writing, or provided orally or by other non written methods.
In OG B 3 - Relationship with Originator, the 2005 rules state that the Originator, upon request, must present a copy of the customer's authorization to an ODFI for use by the RDFI. However, the RDFI should not ask for the customer authorization as a normal course of business but only if an exception is expected or has occurred.
So, to answer your question, you as the RDFI aren't required to obtain any authorizations and the only time you need to ask for one is when a problem has occured. In my experience, rarely is the credit authorization ever asked needed.