Warm-up time.

In your tube component especially, warmup is essential to prime listening. The nodes must be burning for a while before the electrons fly across those gaps in a smooth unwavering flow. Keeping the component on all the time can keep your electric meter spinning as it can generate much heat, depending upon the tube type. But you wouldn't have to remember ahead of time to turn it on.

I've never found the need or noticed any difference in playing music during warm up. I come home, turn it on and putz around and then start playing music awhile later. 

It takes my system about 30-45 minutes before it 'gets going'.  I wonder, though, if just leaving the system powered on does the trick, or if you actually have to be sending signal through for it to make a difference.

I have the same question as parabolic.
The only things I keep powered up are my SS phono-pres.
On some sessions I think I hear an improvement after 20 minutes, on some I notice no change. The ones where I do notice a change are usually playing LPs vs digital sources. And usually with a DL-301 vs a Shure V15.

In my experience it’s 20 minutes of actual playing time for a solid state amp.  So I throw on a CD, leave the room/go about my bidness and when I return the system is ready to pay some sweet sonic dividends.