The commission may, by rule, establish levels of assessed valuation of private car companies, using total valuation of a company or the valuation apportioned to any county, below which value shall be considered de minimis. The valuation of any private car company whose total valuation is ruled de minimis, and such portion of the valuation of a private car company found to be de minimis under the commission’s rules when apportioned to a specific county, shall not be subject to ad valorem taxation.