The concept of an agent, authorized signer, or convenience signer exists, if at all, under state law or the terms of your contract with the customer. Read both.
Is it a good idea? Not to me. Poor decisions about IRA's have income tax consequences that the average person does not comprehend. If the customer wants someone to make those kinds of decisions he or she should execute a POA which specifically mentions the IRA.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.