The general answer to your question is that all high-end equipment should be left on 24/7, with a couple of exceptions and a caveat.
There are two reasons to keep gear constantly powered up: (i) gear sounds a lot better when left on 24/7, especially very resolving ultra-high-end gear, and especially, especially solid-state power amps and digital gear, which take anywhere from 12 hours to a week (Naim amps) to reach thermal stability after being turned on; and (ii) when gear is turned on and off, the parts inside heat up and then cool down (and therefore expand and contract), these thermal cycles being harmful over time to the structural integrity of the parts -- it's basic physics (the same phenomenon, along with heavy trucks and salt, helps shorten the life of highways in the Northeast and Upper Midwest). Leaving gear on will shorten cap life a bit (they are constantly being beaten by the A/C waveform), but caps are relatively cheap and easy to replace, and powered up prolongs the life of everything else.
Two exceptions: (i) Class A-biased solid-state amps, because they burn way too much electricity and turn most listening rooms into saunas if left on 24/7; and (ii) tube amps, because the output tubes pass a lot of current and therefore see real wear if left on constantly. As for gear that uses small signal tubes such as tube preamps and tubed CD players, they should, contrary to common wisdom, also be left on 24/7. They pass very little current if left on 24/7, but die prematurely when turned on and off because of thermal cycles and because most tube gear uses solid-state rectification, which basically kicks the tube in the nuts every time the unit is powered up (that is why some gear, e.g., CAT preamps, have a "soft-start" feature that applies current slowly and progressively to the tube filaments upon turn on). If a piece of gear uses tube rectification (most does not), it's a tougher call - tubes in the power supply can see too much juice to be left on 24/7 and thus may die prematurely, and tube rectification powers up the filaments in the tubes in the circuit slowly (like a soft-start mechanism), so the voltage rush problem with power up is not present.
Caveat: whatever type of gear you have, if your house or apartment is prone to electrical surges or electrical storms, you have to completely unplug your gear to protect it. For this reason, I now use my tube amp from July to September, and my solid-state amp at all other times (as it needs to be powered up 24/7).
It's not green to leave things turned on 24/7, but like auto racing, this sport is not very green.
