Well, the MEL does NOT apply after dispatch but you would probably refer to it. A single bleed MEL'd doesn't actually cap the flight level to FL315 unless your speed brakes are also inop. So in other words if you have a single bleed fault after dispatch and your speedbrakes are working then there is no need to descend. If your speedbrakes are inoperative then if you choose to refer to the MEL you might decide to descend to FL315 but are not obligated to.