Do you mean, "When should you teach that 'ain't' is not acceptable in professional English? I associate 'ain't' with Black-American Youth Idiomatic Lingo.
Please note that I am not being judgemental - only observational.
When you use it, I think you define yourself as a particular segment of society.
My generation of White Anglo Saxon Canadian associates 'ain't' as reflective of being under-educated and in a job interview position would probably not hire the user.
The word 'ain't' is an incorrect contraction of the present tense verb 'be', e.g. 'I am not' and I teach that the correct contraction is 'I'm not'. I've also heard it on TV used in 3rd person - 'She / He ain't instead of 'She / He isn't'. Many people also use 'ain't' as the incorrect contraction of 'I do not have' - as in "I ain't got' and I teach that contraction as 'I don't have'. Note that even 'I haven't' is also really an incorrect use of 'have' in the negative. Correct English regular negatives mostly require the auxiliary 'do'.
I have to disagree with your assertion that "I haven't" is incorrect. It is an old form from before the introduction of the auxiliary "do" into the language; however, it is still very much in use in several dialects of English including to some extent BrE.