Rebalancing: Governance Tool or “Soft Timing”? How do you handle portfolio drift?

I’ve been thinking about rebalancing lately, especially as portfolios naturally drift over time.

A lot of people talk about it like a “market call” (sell what’s up, buy what’s down). But from a process standpoint, I see rebalancing more as governance—a way to prevent unintended concentration and keep risk aligned with the original mandate.

What I mean by drift:
A portfolio can start diversified, then one winner grows into a dominant exposure without anyone explicitly choosing that outcome. The risk profile changes quietly, and by the time volatility returns, the sizing has already become the problem.

The part I’m curious about is how professionals draw the line between discipline and timing.

How do you decide when to rebalance in practice?

Do you rely on:

• threshold-based rules (e.g., X% deviation from target weights)
• volatility or correlation changes
• time-based schedules (monthly/quarterly)
• risk budgeting / factor exposure caps
• something else entirely?

Also, when do you avoid rebalancing because transaction costs or tax friction outweigh the benefit?

Not asking for trade ideas—more interested in frameworks and real-world process.