"[If] employees contributed to the cost of the group medical insurance plan, they are entitled a percentage of the rebate equal to the cost paid by the employees.... [In] deciding on an allocation method, companies can weigh the costs to the plan and the ultimate plan benefit as well as the competing interests of participants or classes of participants provided such method is reasonable, fair and objective.... Generally, ERISA plan assets must be held in a trust. However, if the premium rebate is allocated within 3 months of receipt, companies are permitted to rely on an exemption to the trust requirement."