Should an audio system be left on at all times?

My system is turned on only when in use. If I listen without waiting 15 minutes or so for warm up the sound is degraded during the first couple of songs. My experience says after 30 minutes there is no noticeable improvement. I think the rule of thumb should change.

When turning equipment off how far off does it go? Mine goes into a standby mode. If I turn on my cdp and eject a disc that has sat overnight the cd is warm. I'm figuring it's already warmed up. I believe the speakers require some movement for warm up more so than the electronics.