I've stopped posting on CF, my profile explains why, but I wanted to answer your question: I charge to 100% SOC every 30 cycles to calibrate my coulomb counter and, more importantly, to prevent memory effect from degrading my batteries. As you assert, just balancing the cells could be accomplished much less frequently.
Beware, don't fall into the trap of believing terminal voltage alone can be used to determine when to stop charging. You can overcharge a LFP cell with a terminal voltage as low as 3.4 volts. That's why I count coulombs in and out instead. Here's a good write up on the topic of SOC versus terminal voltage. Skip ahead to the section "The Relation Between End-of-Charge Voltage and State of Charge".