http://www.rcoco.com/sdoc/data/pdfs/...ironmental.pdf
In the Lear 40, like the LR35, the bleed air switches control both the ability to shutoff the bleed from an individual engine, as well as emergency bleed from that engine. They bleed valves fail to the open position with loss of DC power.
Bleed from the powerplant is routed through the shutoff and regulator (mod) valves. Reverse flow is prevented by check valves. Shutting off a bleed isn't done to prevent pressure loss at altitude, though shutting off the bleed is a precaution that backs up the check valve. In normal operation, the bleed air check valves should prevent reverse flow and subsequent pressure and bleed loss into the dead engine.
In flight, one of the chief functions of a bleed valve is isolating an engine which may be contaminating the air supply, or which may be outputting too much pressure or too much heat due to a mod valve failure or other problem. Cycling these valves prior to shutdown does two things: it helps ensure their continued functioning by exercising them (they buildup debris like any other valve, and should be regularly cycled), and it ensures that excess bleed is shut off prior to engine shutdown (which aids in engine cooling).
In the Lear 40, shutting off the bleeds also shuts off the ECS control valves, thus exercising them. These valves also fail open, and require DC power to shut them off; intentionally exercising the valves helps keep them functional.
There are occasions when one may need to shut off the bleeds and shut off emergency pressurization. Regularly manually shutting off the bleed ensures the valves stand a better chance of functioning properly when needed, as well as providing a better engine condition prior to shutdown. After shutdown and DC power is off, the valves will open on their own, as a fail-safe function.