always on vs on/off... one of the longest standing debates in audio.
in general i agree that heat is the enemy of components, so with amplifiers that build up higher temperatures it's probably best to turn them off for longevity. on the other hand, i left my Accuphase E-306v on for 14 years and it never had an issue. on the other hand i didn't hear much of a difference between it being on or off, my theory it's my ears that need warm-up when i finally chill in front of the music shrine.

the M22 runs very cold in idle, so i'd be surprised if there's much thermal stress involved. again, there might be a slight performance difference between cold vs warmed up. initially i kept it on all the time because i wanted to put 200 hours of music on the new speakers asap, but now i have turned on the auto-standby feature. some claim that the process of turning a device on and off is more stressful that just leaving it on all of the time.
NAD doesn't provide an official recommendation with the M22. the Creek Destiny Power Amplifier came with the recommendation to turn it off, then again it did run quite hot.