I think that would be a great idea for the more extensive packages that offer paint correction and interior work as the customer would have some documentation of before, during and after pictures. It shows the client how much work and care was put into the car. However, I wouldnt offer it to basic wash/waxes or single single steps though.
That would be a great signature gift. Business card and CD documenting the entire process you did on the car. Additionally you could add small samples of shampoo, QD, and microfiber in a small gift bag. It cost you barely anything and leaves a very good impression for the customer.