My $0.02 and experience on this...
Several years ago, I refused an update to a 2003 Nissan. To sum it up, the engine could stall due to a poorly-made crank position sensor (CPS). Nissan determined it was cheaper to issue a firmware update than replace the all of the sensors. The update lets the PCM cope with bad CPS signals, so that it does not stall, but other side effects (tach stops working, etc) remain because the root cause (CPS) went unaddressed. I declined the update, as I had already taken the initiative to replace the bad CPS with the updated sensor myself. It resolved all issues.
Fast forward a couple years. Due to other mechanical design/manufacturing defects the engine failed (QR25DE - prone to pre-cat failure and power-valve screw ingestion, causing scoured cylinder walls then ring failure) @ 59K miles. The vehicle was well-cared for, unmodified, etc. When it came time to make a warranty claim, several dealers flat-out refused to help me, many citing the unaddressed firmware update.
That might not have been legal, moral, or business-smart (retain me as customer) but point is - if you care about your warranty, refusing an update is opening the door for future hassle, should something (even unrelated, like my issue) happen. Apparently being an informed/educated/opinionated owner isn't an advantage.