Usage is increasing for local searches on the internet, including areas like North Wales. So what are the geographical terms that local people use? This depends on what people are looking for. For example, if you were looking for a plumber the search is more likely to be more local than if you were looking for a Conservatory. North Wales is made up of a lot of small towns but you would expect to find a plumber in most of them, so "plumber ruthin" may well be a reasonable search term. Those people who say that people wouldn't use the internet to find a plumber should check the latest Google keyword figures. According to Google the search term plumber is used on average 201,000 times. My guess is that a location is probably used in conjunction with "plumber", would you look for a plumber nationally???
But if you were looking for a Conservatory the search term would be wider. Most people wouldn't expect to find a conservatory in their home town and would tend to widen the search. I believe that "North Wales" is likely to be used more than the County. Speaking as a North Walian myself it would be my preference and the figures bear me out. According to Google the search term North Wales is used far more than Flintshire, Denbighshire, Conwy and Gwynedd put together.
So if you're looking for SEO North Wales and you are selling a reasonably high value item then the search term North Wales is probabley one of  the best areas to target.