Engine cleaners can dislodge sludge and deposits in the oil galleys and crankcase sending them through the engine to be redeposited elsewhere, like in the pistons drainback holes. As to balancing, I would be more concerned with the reciprocating part of the equation but not much due to the fact that you are drilling the same size and amount of holes in each piston and the fact that they are so small as to be of negligable effect on the rotating assembly. We're not talking 10,000 rpm race motors here...