I heard a variety of dialects. I would have to say that all are correct for the area they are used in. Some are also very difficult to understand for someone not used to hearing it spoken. When I was in boot camp I literally had to play translator for one guy in my unit since whenever he got excited he talked so fast with his North Carolina accent that no one else could understand him. Having lived there for a good part of my teenage years I understood him perfectly.

For several years I was one of the network admins for a global company, there were times I would get calls from South Africa, Germany, China, Australia, Great Britain, etc. I have to say of all the accents and butchered English that I heard spoken, the ones from Great Britain were the most difficult to understand. Not sure what the term for the slang/dialect version of English that they used, but I found it the hardest of all to understand.

When I write dialogue, I do try to keep it as standard as possible but if the character in question would use "Y'all" instead of "You all" then you'll find that in the dialogue. Or if the character is emphasizing syllables purposefully either because they are illiterate or talking that way for other reasons then I feel its necessary to do so in the dialogue. It's no different than someone writing out "Mommmm!" instead of "Mom", which is widely acceptable.

The same would go for a story set deep in the Appalachian mountains. You would never hear someone say, "Go get that Two and one half gallon bucket and bring it back to me." What you would heard them say is, "Go fetch that peck bucket." or even, "Go fetch that there peck bucket." signifying a specific bucket to go fetch.

Yes it's not proper English but when it comes to dialogue it's very rare to hear someone speak proper English. And it adds a bit of realism to the story when you can do it without it becoming clumsy to read. The trick is not making it hard to read.