Living my whole life in Arkansas, US, I'm certain that "if I were" is never used by locals. Instead, phrases like "if I was," "you was," and "they was," have replaced their equivalents in other regions. I've heard these so often that I think it's necessary to ask if they are grammatically correct as a part of a southern dialect. Does their appearance in a certain region as acceptable excuse their application in formal writing? Personally, I indeed believe that anything not found in generic, "accent-free" areas should not be considered correct. But officially within a southern US dialect, are these constructions grammatical?
This question is the stuff grammar wars are fought over.
Personally, I don't think I've ever heard anyone say "if I were", and I've lived all over the US except the south. The lay idea being, you would say, "I was a king," not, "I were a king," so why should you say, "If I were a king," instead of, "If I was a king"?
This is called the "subjunctive mood" and is used to indicate that the statement is untrue or wistful. "If I were..." or "I wish I were..." are the typical structures for "subjunctive mood" statements.
So, is it now grammatically correct to dispense with the subjunctive mood? That's a matter of opinion, but if you ask most people they'd probably say, "What's a subjunctive mood?" If I was braver, I would voice my preference.