Tube power amp - keep on continually?

preamps are good left on continuously for best performance but power amps isn't a good idea the consumption of electricity is pretty high and the effect on the life of the tubes just doesn't justify the performance gains. Besides 30 minutes it takes to reach full performance isn't really that big of a difference anyway.

Sean can you explain cool down time before shutting the amplifier off? Like how long to wait? Never heard about this before? Would this apply to a class A triode tube amplifier also?