its ok to leave the top one off but leave the others on. The thing is this, even a piece of plastic finding its way into the belt will cause it to jump time. Ive done it myself on a damn miata.
brake the cam sensor pulling the vave cover and a small piece of plastic fell onto the belt. When I cranked it over it jumped about 5 teeth and backfired loud enough to send the office guys running out into the shop to see if I died.
On a zetec, you wont bend valves if it jumps but imagine the hassle of driving along and some piece of road debris flying up into there and wrecking things.
On a related note, a friend of mine left his top cover off after I put news camshafts in and a new belt. a few weeks later he rear ended an SUV and pushed the hood up. since the cover wasnt there, it cut the new belt.
