Looking at various maps, Cumberland county is sometimes included in East TN, sometimes not. I also subscribe to the time zone theory.
Although several counties border NC, Johnson county is the easternmost county.
I was raised in Kingsport and have always offered East Tn as my home and Northeast TN to those willing to zoom in a bit.
In Texas, I just say I grew up close to David Crockett's birthplace. At that point, human decency would expect them to bow down but they never do. These Texans....their memories are probably much shorter than their joy prongs.