Maybe it's not specifically mandated in PCI standards, but I've seen it in PCI questionnaires, and banking institutions such as Wells Fargo (which we use) usually require it, or charge extra fees when you don't. It is certainly considered good practice. But I'm more concerned about the affect on the customer experience. Almost every day (at least multiple times a week) we have someone email or call in unable to place an order online because they are not used to dealing with a shopping cart like this. There is no doubt we are losing sales because of it. For most people, virtually every time they buy something online, they fill out billing and shipping information.