my machines are powered on only when I use them, when they're not in use they're turned off.
Saves money on my electricity bill, reduces pollution, reduces wear & tear on machines.
Turning computers on/off doesn't hurt them from what I know - been in this "business" for almost 20 years. I have however seen power supplies that have failed on units that are left on all the time, my brother-in-law used to leave his desktop pc on all the time until his power supply failed and started to smoke up & spark for some reason, filled the basement recroom with smoke, my in-laws called me over, they were afraid to turn it off because it was making weird noises and filling up the basement with smoke. Suffice it say by the time I got there, the damage was done: the smoking power supply was still providing some form of partial power to the pc and in turn smoked the mb, memory, harddrive, video, audio, network cards, etc. When we powered off the machine (and aired out the recroom), I opened up the box to reveal a dusty machine that was covered in dust & pet hair. The pet hair may not be a standard thing in most homes but dust is inevitable and leaving these machines on 24/7 just compounds the problem. Judging from the bulk of machines I've ever had the "pleasure" to service, most people never dust out their machines - leaving them on all the time turns them into dust magnets. The build-up of dust & dirt in these machines has detrimental effects: dirt & dust (& whatever else is floating in the air) constantly gets sucked inside the case of your pc - and eventually clogs both the pc power supply and fans. As it accumulates, the fans have to work harder to perform the same job, after a while the fans can slow down or fail completely which leads to overheating inside the case (your pc's cpu runs slower as it runs hotter), overheating also can cause other hardware failure and can lead to data loss, dust building up on your motherboard & other hardware components can build up a static electricity charge and one random discharge will fry your motherboard and probably other components as well (have you backed up your data lately?).
You can alleviate these problems by regular dusting the interior of your pc with compressed air (you can by a can of compressed air for pc cleaning for $5-$10 from your local pc shop) and turning off your computer when it's not in use.
You could also be one of those lucky people who leave their pc's on 24/7 and never ever experience any problems (except for above average electricity bills).