We always recommend asking for permission to take photos. In some place people ask payment for any photo taken and the amount is to be negotiated. We suggest consulting the guide to give donations or anything else to school and other organizations. As a matter of courtesy, permission should be sought before photographing individuals and in many parts of the country, particularly among the Afar and among the ethnic groups living by the Omo River, people will demand a fee.
In some sites (Blue Nile falls for example) there is a charge for video photography. You can usually take photos celebrations in Copt churches, but we think you should maintain anyway a distance of respect and sought a photography request beforehand.