Is there a best practice for handling customers or partners?

The answer depends a great deal on the person’s jurisdiction. For customers, it is generally true that because of your previous business relationship, under most laws, you would have permission to contact them related to their purchase. But that permission would not necessarily extend to unrelated marketing. And for partners, the same would be true. The best practice is to include clauses in your customer and partner agreements that grant consent. Always consult with your attorney on your policies and the law.