As usual,the answer depends on the conditions;if the equipment in question get real hot under usage,( the first thing that comes to my mind i a class A poweramplifier),then there is several thing that happens inside it. When turning it on,the heat that is built up causes tiny movments in components,solder joints,etc,which with time can cause problems.On the other hand, as David Belles rightly states,leaving the amp on, can cause electro- lytic capacitors to dry out, but on the other hand,the same capacitors are prone to deteriomental chemical changes, when they don´t have a voltage over them. I think;that the cooler things run,the wiser it is to leave them on all the time.Of course there is also an environmental aspect to it, I´m a bit scepical to big con- ventional class A amps. They are very ineffective! If You use this type of gear, a compromise could be to install a resistor (high wattage type) in series with the line rails and a by-pass switch over the resistor,don´t do this if You aren´t competent enough, though.Doing this is only meaningful if Your amp doesn´t have a stand-by mode.And it can be more or less meaningful even if there isn´t a stand-by switch, and that depends on the construction, and particulary the use of voltage stabilization.