Teaching slang

[Bergbau]

It will depend largely on the needs of the students. If they are likely to encounter spoken slang during L2 exposure then yes, the more common expressions might be useful. Such expressions should be chosen with geographic distribution in mind as some usage is restricted in this regard. Students unlikely to be exposed would benefit more from ad hoc explanation.

[Horsa]

It depends on the slang in question. Most is ephemeral and so not worth the effort. The problem is that native speakers use slang a lot and visitors to English speaking countries find it hard to avoid. Since most people learn a language for communication a certain amount of slang - the more durable kind - probably should be taught. I am not referring to swear words as I have never needed to teach them - most students know them before they can string a sentence together. Indeed here in Russia I am often tempted to correct the spelling of the graffitti written in English

[GaryEM]

For beginners, I think slang is too much. For advanced students and travellers , it it important. The trouble is, different people speak different slang. A student once asked me to teach every slang item in American language. This quickly proved to be impossible!
Slang is something you have to learn as you go. That's how we learned it as children, in our native languages.