the pistons should not have been put on the rods prior to balancing. the piston/ring/pin assembly is weighed and matched. the connecting rod big (plus bearing) and small end are weighed independently and matched. the small end is reciprocating the big end is rotating. the whole point of balancing is to do a dynamic balance. weight matching the parts doesn't really get you much without spinning the crank with bobs on it and making the necessary adjustments. i would think trying to match the pistons with them on the rods is just about impossible to do accurately. i would be suspect of any shop willing to say they can properly balance that assembly.