Liv­ing my whole life in Arkansas in the United States, I’m cer­tain
that *if I were* is never used by lo­cals. In­stead, phrases like 
*if I was* and *you was* and *they was* have all 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 U.S. di­alect, are these con­struc­tions
gram­mat­i­cal?