Leave tube amps always on?


Mark, the owner of Linear Tube Audio, suggested I leave my tube amps on all the time 24/7 if im listenting every day, and to turn them off if not using for a period of time, a few days or for example we go away.  His reasoning was that it uses considerable more energy to turn on/off on a regular basis. 

Do others agree?  Kinda risky since these NOS Mullard tubes ain't cheap and will soon be gone forever.   
aj523
The way I read it is his reasoning is based on environmental concerns and not the life of the tubes. I have had a Cat preamp for over 25 years and there is no difference in sound after 20 minutes of playing no matter how long it is left on. Based on that, I see no reason to leave them on for a minute more than when you are done listening for the day.
Leaving a tube amp on 24/7 implies that it will be left on unattended. If one of the tubes fails and the amp is unattended this could cause a significant fire hazard IMO. I would NEVER leave my tube amps or preamp on 24/7 under any circumstances.