should i turn off the PC every night

I leave mine on and am part of the "power on and off is the most stressful part of equipment life" brigade cheers

I'm of the next day rule: If you are going to use it the next day leave it on If your not going to use it for 2 days turn it off

LED's have a ridiculously long life so don't worry about them. Just make sure the computer is on a battery backup so it doesn't suffer any spikes from a power failure or brownout. I keep my computer, 2 screens and all external drives on an APC unit and never had a problem. A while back I accidentally had a firewire drive on another outlet and left it there. A few days later we had a huge thunderstorm and I turned off everything(except that drive) and it got toasted. Good thing I had it all backed up!