Well it really depends on who you ask. If you ask 50 tech people 25 will say leave it on and the other 25 will say put it to sleep or even shut it down when not in use.
I'm a firm believe to just leave it on. Computers are made to run. Thats what they're designed to do. Cooling isn't an issue. If it was an issue, it would be an issue while using it as well. Heat just doesn't build up all day long. It gets to a point and stabilizes. Thats what the cooling system is designed to do. The issue with leaving it on all night long would be something like if a fan died or something and then it would overheat. Normally fans will give you warnings before they go, like vibrating and things like that.
Also, the motor in the HD will only handle so many spin ups and spin downs. Its really hard on them to spin the disks up and down all day long.
You can put the screen to sleep. Thats not an issue as much and can prolong the life of it. Screens only last so long as well. So if you were to put some things to sleep I would put the screen to sleep. Thats what I do with my PowerMac G5. I just leave the tower running and turn off the power to my display.