It is true that some manufacturers balance brake components, but most--especially price-point brands--do not. Poor casting technique and even sloppy machining can result in a wildly imbalanced component. If left unchecked, an imbalanced part can cause a vibration that will rapidly wear components.
Consult your favorite business or phone directory for a few local and reputable machine shops. Ask who they recommend for balancing. High-performance engine builders in particular develop close bonds with their balancing sources.
Even previously balanced components can benefit from a finer state of balance. Not all balancing jobs are alike; parts manufacturers may balance to tens of grams whereas balancers who cater to performance engine builders are used to going to fractions of grams (though such a fine balance may prove wasteful for something like a drum). It may sound insignificant, but speed can amplify the most modest imbalance.
You may be surprised to find just how common imbalanced brake components are.