Hi Richard,
I think it depends what you want the information to tell you. Most Registration Districts appear to include several towns which are currently much larger than the place the RD name is based on. It is not quite so severe an example, but Trowbridge in Wiltshire has been within the RD of Melksham, which is smaller. Wiltshire is of course the County of Wilton (Wiltonscire), which is a little teeny suburb of the town of New Sarum, otherwise now known as Salisbury. I myself am sitting at a desk in Ystumaner, but you will not find that on your UK road atlas. To find me physically you need 'Tywyn', (in Meirionydd, not Gwynedd, although it is in Gwynedd physically); but to find Church records about me, such as a will proved here, you need the Deanery name, or RD name. It is always a dilemma knowing which 'address' to use, and a most accurate one including all the relevant area names is often not available in drop-down boxes of standardised locations. Many places 'moved' several times too, from one county to another. Even from one town to another (202 Barry Road, used to be in Cadoxton; for a while it was considered to be in Barry Docks; now it is in Barry. The last is illogical, since it ought to be on the road TO Barry from somewhere else. In the Valleys, there are even terraces of houses where one house is in one town and county, and the one the other side of the party wall is in a different town/county, but the same Church court area. Part of the fun! In my records, I would include both Folkestone and Elham in the address.