In general, my recommendation is that people do what suits a story or character. If the character would use grammar that isn't standard, they should. If they're narrating the story, then the narration ought to reflect the way they speak. If it sounds natural for the character, then it's successful, and that's what matters to me.

I agree. I really dislike it when people sacrifice characterization to grammar rules. It can make the characters sound stilted and artificial, especially in dialog. The grammar has to make sense for the character.