Left alone, a 60/40 portfolio becomes an 80/20 portfolio after a long bull market - which means your risk quietly doubles right before it matters. Rebalancing is not about improving returns. It is about keeping the risk you chose.
We favour tolerance bands: rebalance when an asset class drifts more than five percentage points from target, checked quarterly. That trades less than calendar rebalancing in calm markets and acts faster in violent ones.
Do it in tax-advantaged accounts first, use new contributions to buy the underweight asset, and harvest losses in taxable accounts when they appear.