I was born in South Carolina, raised in South Carolina, and live in South Carolina. I'm just as "southern" as any other native here, but I always cringe when I hear the rednecks here use the term "yankee" as an insult to Northerners either visiting or living in the South.
This basically is inherited from the Civil War days where Southerners were taught by the political bigwigs at the time to demonize their brothers and sisters up north. We are all Americans, plain and simple. And to make another point, internationally we are all "yankees" or "yanks" to non-Americans.