I skip out slang. Especially when playing Scottish Characters (As a scot myself.) I've done it once or twice but then people from abroad just criticize me for doing it wrong, which I always found funny.

I think it doesn't matter, it does help though everyone gets their own ideas on what someones character is like in their heady anyway. I think it is all right for a player to give their char a voice but each person will still read it differently. An example of this is the Harry Potter books, everyone had a different view of Harry before the movies came out that isn't specifically Daniel Radcliffe(I know mine wasn't.) The only time it is all the same is when a book is made FROM a movie or a TV show. I kinda lost my point in there somewhere but basically I think it doesn't matter, each person will read it slightly differently and get their own picture in your head no matter what you do, it is inevitable.

On the matter of accents it does annoy me when people generalize. It is something you need to confront when travelling. "You don't sound like this other Scottish person!" "That's cause he's from Glasgow and I'm from the borders..."