Deemed service: appearance or cooperation in assessment proceedings bars later objections to notice validity. Where an assessee has appeared in proceedings or cooperated in an inquiry relating to an assessment or reassessment, any notice required by the Act shall be deemed duly served and in time and the assessee is precluded from objecting that the notice was not served, not served in time, or served in an improper manner, except where such objection is raised before the completion of the assessment or reassessment.