The Illinois Department, Public Aid Committees and Commissioner of Appeals shall not be bound by common law or statutory rules of evidence, or by technical or formal rules of procedure, but shall conduct their hearings in such manner as seems best calculated to conform to substantial justice and the spirit of this Code. They may make such additional investigation as they may deem necessary, and shall make such decision as to the granting of aid and the amounts thereof as in their opinion is justified and in conformity with this Code.