When would you say "I seen it."

"I wouldn't." (That's the answer you'd get from most people here, I bet.)

I, as a Texan, will--in informal contexts and/or when talking with those whose main dialect is "Texan" or "Southern" (I don't think they're the same thang)--say I seen it with absolutely no tinge of embrassment or abasement when, for instance, a fellow Texan/Southerner "axe" me "You seen this movie yet?" and the question does not require the response I done seen it.

I also say "I might could" and "I'm fixin to."

"I've seen it" corrects the grammar and does not sound pretentious. Go for the subtly correct middle ground!

Whether and when to correct someone else's grammar, on the other hand--that probably requires a separate question. For one thing, you're getting into "correct" vs "dialect" and that is a very gray area. I would not walk into that quagmire--either the theoretical or the social one, but especially the social one--without a very good reason.

If you have employees that might embarrass the company when they are offsite, maybe. It's a hard question when one man's dialect is another man's incorrect grammar. I used to cringe when my very intelligent quantum physics professor pronounced "math" with an "f" at the end, because it made me embarrassed for him. He also used the "nook-you-lur" pronunciation of "nuclear". In my opinion, someone close to him should have had a gentle conversation with that man well before he received his PhD and told him that he was likely to come off as less educated than he was by using those pronunciations.

But I would not even think about doing that with an employee/coworker unless I really, really knew them well enough to know that they were not likely to sue me/the company for discrimination over it.

IF for some reason you feel you have to bring it up, find some way to put all the negative spin on the hoity-toity Yankees you are going to meet with. Tell them that there are certain Southern phrases that will kind of grate on the ears of these inexplicably picky northerners, so, for the sake of their pansy sensibilities, we have to practice how to speak Northern when we're up there.

But, like I said before, the question of when and whether to bring something like that up probably needs its own question.