I simply use the county as written on the certificate. I work on the principal of KISS. (Keep It Simple, Silly)
And since most research sites will give either county where it's doubtful, as most will give alternate spellings of names, I haven't had many problems.
It's only when I don't know stuff, since I live in Australia, that I have problems. Like London/Greater London/ Middlesex etc matching up to particular dates. It's fiddly, but I get there in the end.
I learned long ago about Wales/Shropshire/Salop. I have a few ancestors from that area. It was confusing to say the least.
And thanks Plummie, your reference to Gloucestershire/Somerset has just solved another one for me.
Leonie.