Balancing an engine

I have small blocks so I figured I might as well ask this question here.
We all know that our engines came from the factory with the rotating assembly just bolted together. No special balancing.
My question is. Under what conditions is it required to get the rotating assembly balanced? ie: over a certain rpm, over a certain HP

Jack
Take it from me and look at my threads. Balance the motor at all costs or you "may" regret it. Whatever it takes!