The front motors are perfectly horizontal, but the rears are both tilted outwards slightly, perhaps 5 degrees positive camber. I'm sure this is a design feature to aid yaw control too (vectored thrust with induced torque).
on the p3 the motors are supposed to be tilted out some. That helps it have less VRS and also makes it fly way more stable as it puts some outward pull from all 4 arms.
THO on all other models other then the inspire 1 and the p3 the motors are not supposed to be tilted. BUT I would suppose that putting a shim under all 4 would be a good mod for p1s and p2s providing you can also keep the motors from rubbing on the sides of the holes in the top of the arms. and they also would have to be so that all 4 all tilt the exact same amount or at least very close to exact.