In my head 'North X' means it's a separate settlement, whereas 'X North' is just the most northerly station of a settlement.
For example; West Malling and East Malling stations are located in separate villages. But Maidstone West and Maidstone East are both located in the same settlement of Maidstone.
Obviously this isn't a hard and fast rule, but seems to work in most cases.
Your rule works very well on the basis that there are lots of places where N/S/E/W look like prefixes - but really they aren't, like East Grinstead (no West Grinstead :P) or North Road.
East Garforth, North Llanrwst, South Bank and Allens West are some examples that create a problem in both senses. As far as stations go, there are actually quite a lot.