I've had my PC on for days at a time when I got it and was downloading/installing the OS, programs, games, and transferring files. It can handle being on all the time, but I still turn it off.
Basically even though I use it regularly I think it'd use more power being all all the time, and sometimes I don't use it for days at a time. Primarily the reason I turn it off is overheating. Not that the PC will overheat (current CPU temperature is *Checks* 39 degrees, and that's perfectly fine), but rather that my room is quite small and with the PC running and me in bed it can get quite warm (particularly during summer) so I turn it off. Also the fan noise is distracting at night as is the power light.