Do you always keep the Amplifier powered on

That's true it wastes electricity but so does making coffee, watching TV etc. It is for me a matter of convenience. With high power class A amp it might be a different story.

If you are making coffee on an aircraft carrier two hundred gallons at a time could be, my 5 minutes for 2 cups not so much.

My espresso is about 1500W for 20min (minimum to warmup machine) making 0.5kWh per day (for just one coffee) while my amp and the DAC take together 20W for 24hrs = 0.48kWh. Landscape lighting takes much more, not to mention driving SUV, watching plasma TV or listening to class A amplifier. The most energy wasteful thing you can do is having a child. How many do you really need - one should be enough. :)

Overpopulation is sad and it's likely too late to turn the ruination of the planet around, so I've changed my stand on this issue and say LEAVE IT ALL ON as it no longer matters. Sort of sets you free doesn't it?

No. I kept my Boulder 1060 powered on when I was home on the weekend & found the amp was 95+% warmed up after 2hrs (ie: good enough), otherwise I left the amp on standby. With my current Vitus SIA-025. I find myself in agreeance with Chris Thomas's comments in his Hifi+ review -

"From stand-by, straight into class A I would say that you are looking at a half hour before it really begins to sing and then it just seems to keep getting better. Even after a solid afternoon and evening's listening I would swear that the amplifier was growing more fluid, textural and resolute with better dynamic contrasts, tonal shading and the delicacy of its musical message just seems to become more profound."

To summarize, there are no hard and fast rules. The benefits of keeping a (solid state) amp switched on all or most of the time vary from amp to amp. But rule of thumb, large amps with oversize power supplies and filter banks will generally benefit more from being left on as it takes the psu's longer to warm up and reach optimal thermal conditions.