I have 2 computers which run 24/7. One I use for working on side-projects/work and the other I use as a web server, ftp server, streaming audio, torrent downloads, etc.
In the past I've always left 2 sets of computers on 24/7 as well, but I used to have problems with hard drives going bad. I later figured out that the reason this was happening was that I had them set to never turn off when idle.
So if you have your PC up all the time, make sure you have your hard drive(s) set to turn off after a certain period of idle time.
I'll be technically leaving my system on, but in sleep mode. A deep sleep mode. The mode where the mouse and keyboard recieve no power, the fans turn off, the led lights turn off. Even the CPU fan goes off. Just for the sake of 3 second boot times and continuing where I left in applications.