I generally leave them on as I listen to them everyday. If I'm going to be out of town for a few days, or know of a storm coming... I turn them off.
The front panel switch does not actually turn the whole amplifier on/off...
When plugged in and switch in "mute" position, the power supplies and input stage are up and running. This keeps the low current stages warmed up, and they use very little power.
When "play" is selected, the output stage is turned on.
If you want to turn them completely off, you can connect them to a switched outlet or power strip, conditioner, etc with a switch.