Leave it on?

There is no one size fits all answer. Some equipments turn themselves off after being inactive for a certain period of time (Marantz AV-10), others are just impractical (Class A amplifiers). On the other hand some can stay on and ready without causing any damage to themselves or the environment (Class D amplifiers, DACs etc). Some manufacturers recommend it. Just make sure the equipment is in dark mode (display off), otherwise you might cause display burn in where the characters stick to the display like a watermark even without power (not good for resale value). 

@mylogic 

Your logic is flawed at best. 
“How confident are you leaving other appliances on all day and night?”. 
Do you turn your fridge off at night?  Leaving a DAC on with the display off won’t affect global warming in a million years. There is no work done and the energy consumption is negligible. Comparing this to a car engine in idle state is ridiculous. The engine is doing serious work so much so that the fan will come on after a certain time to cool it off. The engine is still running even at idle. 
You are fast to call people ignorant and unconcerned about the environment but your logic doesn’t make sense. Why would a manufacturer give you advice that will be detrimental to the longevity of your equipment and expect that you will buy another one from him after the first one fails? Do you have the stats on the number of fires started by hi-fi equipment left on continually?

you should try to express your opinion without throwing insults around like you are a nine year old.