My money is on the regulator. It can leak down, and the fuel remains within the system. If an injector was leaking, then the car would run poorly because it would leak at undesirable times. Think about it, the injector will leak when there is pressure on the system, it will result in the cylinder running rich where the leaking injector is. If the regulator leaks down, then you would have trouble with any starts, not just warm ones. You said that there was no fuel coming out of the Schrader valve with engine off, I would wonder about the pressure regulator first.