I leave everything on standby that is solid state, but not the pure class A Threshold T-200 - it takes about 20 minutes to warm up and start to sound good. The outboard power supply of my Threshold DAC-2 runs extremely hot, but I leave it on anyway since the manual recommends it - and there is a noticeable difference after it's been on for about 72 hours.
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?