I think you need to think about who your potential readers are too. For instance, if you think your book will appeal to an international readership, you may not want to make it too culture-specific. A certain accent or dialect may be loaded with meaning for an English person in a way that it isn't for someone from, for example, the U.S. or Australia. What I mean is, an accent/dialect may immediately 'place' a character for you in a certain region/class/income bracket, but for someone beyond the British Isles it will probably just mean nothing more than that they have a quaint regional 'British' accent.

The further afield you go, the less people will pick up on the subtle nuances of speech that divide people. I'm Irish and I think I have a broad knowledge of regional British accents. I would have a rough idea (I think) what a Geordie would sound like (Jimmy Nail??), but I wouldn't be able to distinguish between different classes within that accent (though I suppose a rule of thumb is the broader the accent, the lower down the social scale the person is). Similarly with Scottish accents, I would recognise a Scottish accent, but wouldn't be able to tell Edinburgh from Glasgow, working class from 'posh', etc.