I'm firmly in the turn it off camp:
1) Turning off the PC saves electricity, so it is good for the planet as well as the wallet.

2) Leaving the PC on 24/7 will suck more dust into it.

3) Leaving the PC on 24/7 will cause the mechanical parts (disk drives, fans) to fail earlier - all bearings wear out.

4) Leaving the PC on 24/7 (and especially a CRT monitor) is a potential fire hazard.

5) A lot of the unreliability that used to be reported in machines that were turned on and off regularly was caused by "thermal creep", where chips would gradually work their way out of sockets. The way modern circuit boards are made renders thermal creep a thing of the past.