Leave it on all the time. It will sound better unless you are getting a Class A amp.
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I made the exception for Class A in memory of the time a Krell customer relations official told me that "If you leave our amps on without playing them you void the warranty". I was a Krell dealer at the time and one of my KMA 100s had just blown up for the second time. I had let it warm up for about 15 minutes before giving a demo. We parted company soon after and I don't think either of us has ever missed the other.