It is the policy of the agency to promote collective bargaining negotiations between employers and the exclusive representatives of their employees. Parties are encouraged to engage in free and open exchange of proposals and positions on all matters coming into the dispute between them. Whether a particular subject is mandatory or nonmandatory is a question of law and fact to be determined by the agency and is not subject to waiver by the parties by their action or inaction. It is the policy of the agency that a party which engages in collective bargaining with respect to a particular issue does not and cannot confer the status of a mandatory subject on a nonmandatory subject.