A reed switch is a pair of metal contacts inside a little glass capsule. When a magnet comes near, the contacts are pulled together and complete a circuit.

I thought that the contacts inside had to be aligned with the magnetic field, but this isn't the case. What happens is that the magnetic flux is 'conducted' by the contacts magnetising them, as a result they are attracted to one another.

This means that for the highest sensitivity the magnetic field lines should be aligned along the length of the reed switch.

It also explains why there is no marking on a reed switch to show the 'top'.