I have a 2004 Mondeo, and the Haynes manual says undo the bleed screw as the reverse flow of fluid into the master cylinder "has been known to" "flip" (Haynes terminology!) the MC rubbers leading to a total loss of braking. I think that's nonsense for various reasons, mainly that when the pedal isn't depressed there's an open route for fluid back to the reservoir. And Haynes doesn't mention it for the front brakes, though the piston can be pushed back much faster than the rear can be wound in. So I ignore it, never any problem.
You can work out which way to turn it by looking at the handbrake lever on the back of the cylinder (inboard on the car). If it turns the shaft clockwise when the brake is applied, the actuator has a left-hand thread, as it pushes the pad away. So to retract the piston it goes anti-clockwise, (to screw the piston on to the actuator). And vice versa.
On the Mondeo it's one way for the saloon and hatchback, the other for the estate, as on one the handbrake cable connection is above the cylinder shaft, on the other it's underneath.