Don't matter

[banderas]

Informal is informal. Slang is highly informal.
I'd rather be calling 'it don't' a colloquialism, though. And I am not saying it's not in use, or anything like that. I'm just saying that it's grammatically incorrect.

I.

"Don't is the earliest attested contraction of does not and until about 1900 was the standard spoken form in the United States (it survived as spoken standard longer in British English)."

It is the way some people speak. Other people (prescriptivists) have decided that "it don't" ain't English, but it is. It is just not standard English (the prestige language). I agree if a non-native speaker asks you have to say it is incorrect. However, between native English speakers there is no correct or incorrect but nonstandard.

[IvanV]

We can certainly discuss what is in some people's opinion English and what's not, but that's why we have prescription, to tell us what is standard and what's not. By saying something is correct(incorrect), we refer to whether it has it's place in standard language or not, and not whether some people use it (we won't be nitpicks in defining the meanings of correct and standard, now, won't we?). I can't stress this point too strongly - linguistic prescription does not dictate anything to anyone. You are not obliged to follow it, but then you have to mind where and to whom you are speaking. As there are levels in society, there are respective levels in (for lack of a better term) quality of one's language.
And after all, I think it's just a personal opinion of every one of us.
As a teacher, it appears to me that the only possibility for me is to follow 'good English'. But it doesn't mean I don't fall off the track once in a while, and it most certainly does not mean that it's not normal. To what extent I stray is another personal preference. And it, again, depends on situation.

All the best! ;)