The system is flexible so that you can actually do both. You may want customers to expire rather than documents / products because if a customer has subscribed to your service for a year then they are entitled to carry on viewing those documents after their subscription has expired. The system prevents them from viewing documents with dates outside their subscription period.
On the other hand, you may want to issue your customers or prospects with time sensitive trials or samples of documents / products because you do not want them to carry on viewing once it has passed its expiration date. In this case once the expiry date has been reached the document / product is no longer viewable.