My rotors were all balanced fine out of the box. BUT, when I broke my first one, and replaced it with one from a new pack? It was NOT in balance with the other, non broken blade. It was waaay off. (different production dates I assume). Anyways, I took two new blades out of the same pack, balanced them, and found that both were already perfectly balanced.

It appears that mixing blades from different packs can lead to an imbalance issue. Just from my experience anwyays