As was pointed out above, the answer is component specific. My ARC 150.2 SS amp is designed to be left on 24/7, and in fact ARC comments that if turned off, it can take up to a week to get back to full "musicality" once it is turned back on. My CDP stays on stand-by when not in use. But my ARC tube preamp is turned off after use, and only turned on when I'm going to listen; ARC's comment is that leaving it on continuously would merely wear out the tubes prematurely.
Do you leave your components on 24/7?
Lately I've been leaving my components on all the time, on the assumption that a) they'll be ready when I want to listen, and b) the on/off cycle ages the equipment (tubes, anyway) faster than leaving everything on. Is the latter a reasonable assumption?