You don't see this as a rule because sometimes ads aren't appropriate or just aren't available. However, if ads make sense for your project, be sure you do the following. Have your advertisers sign a simple contract, specifying the following:
- Ad specs: size; color; bleeds available or not; position. you promised any.
- Artwork specs: file format and size and deadline
- Cost: free, or specific $$s, or barter for goods, services, or other concessions. Specify the limits (e.g., for the life of the edition, for so many books sold, etc.), when payment is due, and any extra charges that apply (e.g., design charges if you have to create the ad for them)