It's not always necessary. There's no detection for drives over LT+, so they should be safe. The only case is a Samsung, which must be re-flashed to stock. This ONLY applies to updates that have new DVD firmwares (13146 is the only so far). I would check with the scene first before every update, just to ensure that you won't get banned.
What iMoses said, there has only ever been one case to re-flash to stock and that was due 13146 update which introduced new firmwares.
People recommend it but there is no evidence out there to support the theory that you can get flagged during the update. Think of it this way LT+ is undetectable thus far, why would Microsoft only look for your flashed firmware during an update?
One thing many people feel safe with is waiting a few days for positive feedback from other members. If you are worried I do suggest looking around the forums when a new update is introduced.