Heard the same throughout college
While getting my EE degree and learning about electronic components I recalled different professors stating that exciting and powering down electonic circuits (transistors, resistors, inductors, capacitors, etc) is ultimately what causes them to deteriate or burnout over time. Same holds true with light bulb filaments.
Every time you turn on and off a component that surge of electricity charges up the components and causes a huge temperature gradient...the same happens (although not as dramatically) when you power it down. Meanwhile keeping the juice flowing in a steady state is far less damaging. If you're not outputting anything to your speakers its probably not costing you that much electricity either.
On the other hand, with the sophisticated design of most of today's top electronics I think you'd be O.K. powering it down as well. I think if Denon and Yammie were really concerned about this they would recommend doing so (or even threaten to void you warrenty if they can prove you didn't!)
"Mr Smith...judging from the strong spring coefficient that still exists on your unit and remote "power-on" button and the age of this unit it appears that you were not powering the unit off after use! WARRANTY VOIDED!!!"