Balancing before charging is good if there is a pretty big imbalance. It gets the cells closer to the same before you even start the charge. If you try to charge and balance a badly out-of-balance pack, one or more cells may get to the CC/CV threshold (3.6v for A123 cells) before the balancer has a chance to equalize the cells. This is more true for external "blinky" type balancers. Balancers built into the charger don't have this problem because there is communication between the charger and balance circuits, and if the CC/CV threshold is reached before the cells are balanced, the charge rate is automatically reduced or stopped until they balance.
For packs in good condition with minimal imbalance, it's ok to just balance while charging.
If you leave any kind of circuit attached to the battery at all times, make sure it has VERY low quiescent drain - something under 100uA (micro-amp). Devices that draw currents of even a low 10mA can totally drain a cell over time. For instance, a fully-charged 2.3Ah A123 pack hooked to a device drawing just 10mA of standby current will be discharged COMPLETELY in under 10 days. But this same pack hooked to a device with 100uA of standby current will be discharged in a little over 2.5 years.