Deemed service of notice precludes later objections where assessee appeared or cooperated, unless objection raised before completion. Where an assessee appears in or cooperates with any proceeding or inquiry relating to an assessment or reassessment, any notice required by the Act for that proceeding is deemed served in time and in accordance with the Act, and the assessee is precluded from later objecting that the notice was not served, not served in time, or served in an improper manner, except where such objection is raised before completion of the assessment or reassessment.