Generally when acting as a custodian, you are not acting in a fiduciary capacity. When acting in the capacity of Trustee however; you are. Carefully read the IRA documents and it should define in what capacity you serve and what if any investment authority or discretion you hold.
If you are an OCC regulated institution, I suggest applying their definition of a fiduciary - "Providing Investment Advice for a Fee"
I didn't really answer your question, but I hope this information helps.