PS Audio P300??

Can the PS Audio P300 be left turned on all the time?

It can, but since it may generate a fair amount of heat, and run up your electic bill. The P300 is only about 50% efficient, so if you have a 50w load (say, a preamp and CD player left in standby modes), the P300 draws about 100w from the wall. But if you are the kind who forgets to turn out lights when you leave a biggee.
Sure, if you need a space heater.
Yes, and you will notice it on your electric bill. IMO if you can afford a system that benefits from a P300, then the electric bill shouldn't be a problem. Most solid state components, especially digital, benefit from being left on 24/7.
Is it common practice to turn on and off the P300 on a regular basis?
I draw about 120 Watts with my front end components from the P300 and I disconnected the fan with Paul McGowan's approval. Sure, it runs hot but I leave it on all the time. It has been running like this for over a year without problem.