they often do power cycle cycle (on/off) testing as part of reliability testing of integrated circuit components, but the number of cycles is probably orders of magnitude greater than people would actually perform in the real world. so i don't think that this is a very good reason to leave components on 24/7.
03-01-11: Elizabeth
Leaving a SS item on 24/7 is better for wear and tear.
i think that, in general, leaving components on 24/7 is a massive waste of energy resources. as many have stated, 15 minutes should be more than enough to allow a component to reach fairly stable operation, especially under drive conditions. electron temperatures are measured relative to absolute zero (-273 degrees celsius), so after about 15 minutes temperature differences (relative to absolute zero) are going to be relatively small in percentage terms. it seems hard to justify wasting energy under such circumstances.
for my own part, i turn off my amplifier and preamplifier but i don't turn off the phono preamplifier. the reason being that the on/off is not located at the front on the phono preamplifier but since the power dissipation is comparable to that of a night light i don't really make such a big deal over it.