I'm just wondering...what if your novel is written in first person, and you have a character who consistently speaks poor grammar? It's one thing to have idioms and double negatives in an occasional line of dialogue, but if it's a first person narrative, that means the reader will have to be prepared to deal with this throughout the entire novel.
I'm actually glad someone has posted this topic because it's been an area of concern for me, as well. One of the MC's in my novel is almost notoriously illiterate--at least verbally--but his narrative is the one that kicks off the book. Most beta readers have found him to be my most charming character (the Huck Finn comparisons come up a lot) but I've also had my share of readers who cringe and moan and slash through every "ain't" and "cause" until it looks like the page is bleeding.
I've had some concerns that opening the novel this way, with this particular character's voice, could have an adverse effect on potential readers. Will it make browsers apt to put the book back on the shelf? Admittedly, I probably know the answer already. As I've seen from my beta reader responses, people either love this character's voice (and are immediately drawn into the story as a result) or they hate it. I suspect there is not much in the way of a happy median here, because the simple truth is, there are some who will enjoy that folksy sort of voice and others who will be put off completely. And those peoples' tastes have already been formed, long before my book was ever written.
But I will say that I have done A LOT of work to edit, tighten and reel in the voice so that now, after more than five years of work, I can safely say the voice is as polished as I can get it without adversely changing it to something it's not. In other words, it is possible to remain true to your character's voice while at the same time editing and polishing for clarity and flow.
The most important thing is that the reader not be confused. A useful trick is to ask your beta readers to read passages aloud. If you find they are stumbling all over the dialogue because the grammar is tripping them up, that's probably a good indicator that you need to make it a little more accessible. (This usually involves, at worst, only a few minor alterations). But if the words flow, and the comprehension isn't affected, there's no reason to change your character's manner of speaking.