I would put it in the Agreement of use so you can at least inform the customer. I would also do it at issuance and to spur use, I would notify the customer a few weeks before termination that it will be turned off if not used.
If the customer forgets about a required use but has it set aside for an emergency, you'll have customer service issues if they have that bona fide emergency and you turned off their card.
Part of the thrust here is to ensure they know where it is, and that they use this convenient access device. Remind them about it if need be. If they really don't use it or plan to, turn it off as it is a liability.
My opinions are not necessarily my employers.
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell