Leave it on?

Turn them off.  Heat and cost matter.  

The exception to this is some Class D, like ICEpower which absolutely sound bad until they have warmed up for 2-4 days.  Can't explain this in any logical way.  

I do not subscribe to that philosophy.    Many devices today have a standby or remote IR sensor so they are "on" sort of.   And if they have a mechanical switch I use it.

My main set up is all tube and that is only on when I'm listening and have time to listen for a few hours 

My second system is Solid State and has a full function remote.  Plus it has Auto Off so I let it do that when there's no signal for 20 min .  Either way it's " off" when not in use 

If it's got an I.R. window its always on .   

I put my integrated amp into standby when done listening. 
My DAC doesn’t have a standby mode, it’s either on or off using the switch on the back panel. It runs cool so I just turn off display and leave the unit on. I unplug everything when we’re going away on a trip. 

If your electronics have a standby mode I encourage you to use it. 

McCormack recommends leaving it on all the time.  I did this and my amp lasted 30 years, and it’s always warmed up and ready to listen to immediately.  Think about it — light bulbs mostly blow out when you turn them on.  Capacitors and other components constantly going from cold to warm just can’t be good. Unless you’ve got a tube or class A amp I’d just leave it on. 

Interesting. I emailed Hegel about my H200 and they said that it is designed to be turned on and off. It is interesting that when I push the power button on the front it takes a second for it to make an audible clicking noise when it powers up. Maybe some kind of soft start?