Think of the social situations in which you would say, "Whaddya want?"
We are so familiar with both our language, AND the nuances of social interaction, that we would know when we might use this for comic effect. Until a learner of a language has absorbed enough of this to judge, he is mainly using language parrot fashion - hence, the number of posts asking for the difference between very similar expressions, some highly colloquial, and some formal. Let then try saying something highly colloquial in a very formal job interview and be judged accordingly!
So, in the same we can recognize those wee dialect words that we occasionally hear, da ya ken, it seems a good idea for students to have them explained if they come across them in watching a DVD movie; but there's enough for them to grasp without making it part of any curriculum.
And where do you stop? What about teaching them muvver, bruvver, wiv and other variants of pronunciation, Bro??