Smokey Yunick in his book Power Secrets- "Frankly, I think engine balancing is anything but an exact science, and you don't need to get upset if things are off a little.
There's no such thing as an engine that is perfectly balanced at all engine speeds. You can determine a recommended balance weight according to the formula that the crank manufactures use, and pick a crank speed-say 5000rpm- to put in the formula with a different crank speed-say 7500rpm- the recommended weight will be different. So, at best you can only gain a "true" balance within a very narrow crank-speed range.
And from what I've seen, this true balance is at best a approximation. When you look inside the oil pan as the engine is running at high rpm, there is a tremendous amount of oil caught up in the windage from the spinning crank and wrapped around the crank throws. This oil has a lot of weight, and as the engine runs, this oil will migrate all over the assembly. Sometimes it hangs around the front throws, someetimes it is in the middle. The pattern is totally random and the size of the oil cloud can get larger or smaller. All of this happens in a test engine sitting on a dyno, so who knows what it's like when the engine is in a moving car and you have all sorts of front, rear and side loads in the picture.
This oil cloud must affect the balance of the operating assembly- and there is no way to account for it- so I little reason to worry much about engine balance. If you have to replace the pistons in an otherwise intact assembly or you have to do some machining or grinding- as is occasionally the case when you change a cam and a little more piston-to-valve clearance- I don't think you have to go to the trouble to rebalance the assembly if you can get the weight on the piston end of the rod within about two grams of the original balance weight. If you can get the replacement weight slightly less then the original balance weight, I'm sure you won't have any trouble. However, if things get out of hand-more than about three grams difference- you should get the assembly rebalanced to be on the safe side.