Lowell,
Balancing takes the weight of the pistons and makes them all the same weight. Then they make the rods all the same weight. Rods are actualy done at both the big and small ends. Weight for the bearings, rod bolts,
oil, all is taken to account for. This weight is then calculated for whats called a BOB weight. The crank is then spun on a electronic machine which will tell the machinist how many grams your crank is off based on the BOB weight given. So having a crank heavier on the front or rear is not important. The weight the crank produces while turning is whats important. A steel crank is heavier then a cast crank, but both can be balanced for the same piston & rods. Lightweight race cranks will often need heavy metal added to the counter weights when balancing.