I leave my PC, monitors, and UAD Apollo on 24x7.
I sometimes remote into my PC from work, so that's why I leave it running. I leave the Apollo on because the firewire drivers were a bit wonky at times and if I shut the interface down but not the PC, it wouldn't always recognize that it was turned back on. It seems to be better with Thunderbolt but I'm fine with my process.
I do press the mute button on my interface when I leave the studio though. My Adams then go into standby mode and I don't have to worry about the interface/pc sending any audio.