What is the etiquette concerning courtesy titles?

Courtesy titles such as Mr., Ms. (or Miss), or Mrs. are always used. Some say the title Ms. (as opposed to Miss) should be reserved for business correspondance and should not be used on wedding invitations, while others say that it is proper form if the woman is divorced or has chosen to keep her maiden name. Please see: Addressing The Invitations for examples on which courtesy titles to use when addressing the wedding invitations.