I always rotate the motor to TDC on the cylinder I'm working on. I wouldn't run any more than about 10-20psi or just enough to hold the valve up. This way if something happens like a sudden loss of pressure you won't drop the valve out of the guide. Running full line pressure can be enough to rotate the engine but isn't enough to allow you to break loose the locks from the retainers if they've been together awhile.
BSE Racing Engines