The Tax Commissioner shall adopt and promulgate rules and regulations to establish standards for the appraisal of classes or subclasses of real property in a county. The standards established shall require that the appraisal shall be based upon the use of manuals developed pursuant to section 77-1330 and shall arrive at a determination of taxable value on a consistent basis in accordance with the methods prescribed in sections 77-112 and 77-201. The Tax Commissioner shall also establish standards for appraisal contracts which shall, among other provisions, require that all such contracts shall require the use of manuals developed pursuant to section 77-1330. No appraisal contract shall be valid until approved in writing by the Tax Commissioner.