If your going to be away from your computer for more then a few hours then there is no good reason to keep it on, other then maybe you have it downloading or doing some other tasks while your not there like checking for software or driver updates or running a defrag or antivirus scan.
what I do is set my screen saver to come on in 10 minutes, the monitor to shut off in 15 minutes and the computer to go into sleep mode in 45 minutes. This way after 45 minutes the monitor has turned off and the computer has essentially been turned off, just with the fans left running. and it can be woken up in 5 seconds. Leaving your computer on, no matter how many components have gone into sleep or hibernate or whatever, will consume electricity and the computer wont be doing anything, so why leave it on?
what I do is set my screen saver to come on in 10 minutes, the monitor to shut off in 15 minutes and the computer to go into sleep mode in 45 minutes. This way after 45 minutes the monitor has turned off and the computer has essentially been turned off, just with the fans left running. and it can be woken up in 5 seconds. Leaving your computer on, no matter how many components have gone into sleep or hibernate or whatever, will consume electricity and the computer wont be doing anything, so why leave it on?