The commissioner may permit insurers to use appropriate systems of schedule rating filed by any insurer or advisory organization approved by the commissioner, subject to rules adopted under RSA 541-A, to assure the uniform and impartial application of such rating. Such ratings shall be:
I. Based on an employer's safety policies and record, including its policies with regard to injured workers' return to work;
II. No greater than plus or minus 25 percent of the insurer's base rates;
III. Voluntary on the part of both the insurer and the employer; and
IV. Applicable to an annual premium greater than $1,000.