When to re-balance a motor?

Another interesting quote:
Little Tolerance for Imbalance
The “old school” tolerances for balancing used to be plus or minus 2 ounces for stock engines (56 grams), half an ounce (0.5 oz. or 14 grams) for street performance and two-tenths ounce (0.2 oz. or 5 to 6 grams) for racing engines.

These numbers provided relatively good results with the engines and speeds that were common a couple of decades ago, but they are not even in the ballpark with today’s stock and racing requirements.