I disable my hibernation mode and all that, I do however keep my screen saver on now, and "turn monitor off after x minutes" since this is a new LCD and I keep it all shiny and good, but on my old CRT I never cared since it was gotten used (from family) and I knew I'd be replacing it.. amazingly nothing stuck on it's screen....
I keep my disc drives on mostly because when I get back and want to be on- I hate waiting the few moments to "rev them up" to get them going again. Not sure if it's so good on em , but they are supposed to be good for so many thousands of hours of use... and yea.. there's only 24 hours in a day..
I don't think the biggest problem is turning them off or leaving them on (other then heating the room or Air Conditioning bills and such) since most computers use a ton less power when on standby- even without the standby or hibernation on. I listen for about 20 mintues then my CPU fan starts to idle lower since nothing is going on, then again I got a brand new system, i7 and all so that could be why.
After a good night I wake my Monitor and my temps are around 35C in a horrible case. Messin with web pages and stuff it'll go up to 40C Games go from 45-70C depending on the game and how long.
Once I get my HAF case things will change I am sure. I wonder how many of you remember the Xbox360 problem was heating up and cooling down cycles which break it ... case setup might have the same effect on your PC, so make sure nothing is tension that doesn't need to be tension, no reason to physically break down your PC before its time.
Biggest thing I worry about is Air Flow to my case. Not turning it off. The only time I really turn it off is when I go to bed and think "Hmm.. I probably won't get on it first thing in the morning." Boot up is still seemingly long for me... when my new case comes and my new GPU I plan to format/reload the entire system and set it up for Prime Gaming.