From a Country viewpoint, Johnny Cash's unique sound and style wasn't done on purpose. It was because that's the ONLY thing the trio could play when they first started. They actually learned to play AFTER they had a couple of huge hits. Ironically, every time they strayed much from the bum-chick sound, they weren't as successful, at least until later years when John recorded with some of the hot rock stars of the 90's. And every note he ever sang was sharp, pitch-wise.
DonM