The prop valve does several things. It acts like a safety device in case you rupture a front or rear line. It will keep the brake pedal from hitting the floor and allow you to come to a stop. It also controls the pressure to the rear brakes so they dont lock up. The prop valve also sends a small amount of pressure to the rears first to act like a rudder and help in slowing down in a straight line. Yes, you should keep it.
