It’s a known fact: people murder the English language every time they open their mouth. I’ve read a few editor interviews in which they stated even though they know people do this, they don’t want to read it. They want dialogue written without dialect or slang.
I often think about those interviews as I’m writing conversation; nevertheless the voice in my head wins every time, and I write what I hear. It may be shoulda, gonna, whadda, but it’s what I hear. These imperfections make dialogue flow and often give you a hint about the character. When I’m writing, those editors’ words nag at me, and I might not drop the “g” off an –ing word or two, but for the most part, I continue to write it as I hear it. And nope, that ain’t gonna change anytime soon.