Some times squeeling is normal with new pads. The dealer replaced mine and for the first week they kind of squeeled. Before you go to the expense of replacing rotors have a mechanic check them out. They can measure them and tell you how much you have left of the rotor. I think you can get down to 60% before you have to replace them. EIther way the front ones are the most important. If you'r not noticing anything wrong with how the car brakes aside from the squeel I would leave it. Maybe when you have your winter tires changed back to summer have your mechanic measure your rotors. If they are bad you can replace them then.
1995 E420 - retired
1986 420SEL - retired