To Power Down, or Not to Power Down

My pre-amp (Rogue Audio 99) and amp (Cary CAD-120S MkII) are both tubed. There are days when I will have a listening session for about an hour followed by a two or three hour break followed by another listening session (or two). If the next session will be within an hour or so I leave the system up, but I'm not sure what to do when the interval between sessions is longer. My questions are...Is it better for the equipment if the system is powered down between sessions that are more than 2 hours apart, or should the system be left up? Is there a detrimental effect on tube life by cycling power up/downs in this short a time period? I would prefer to power down for simple green reasons, but the cost of eight KT88's failing prematurely would be far greater than the increase in my electric bill. I would like to know what others in a similar situation do?
Jerry