Battery Pack Management: need to balance during discharge?
Because charge efficiency of the battery and control circuitry is not 100%, you're just going to end up wasting energy and adding cost for minimal returns. As long as your batteries have the same specifications and are properly balanced prior to use, then your risk of damaging one battery before another during discharge is pretty remote. You're much more likely to damage an unbalanced cell during recharge.

Generally it's bad practice to fully discharge lithium ion and polymer batteries as it results in shorter battery lifespan. So assuming you're only running your batteries down to 50-80% depth of discharge (DoD), the risk of damaging an unbalanced battery is negligible.

To ensure quality of your packs, I would recommend putting them through an 80% discharge test as soon as they're ready. Measure the voltage at that DoD to establish a baseline. Anything below the normal average you'll know has a problem.