Our two shibas have very different tails.
Loki has a very loose curl. His tail curls and uncurls easily and all the time. You can tell when he's fearful or apprehensive when his tail uncurls. His tail seems to have a personality all on its own.
Jujube, on the other hand, has a very tightly curled tail. It's more like a little bundled knob. Perhaps she never gets fearful or apprehensive -- she is pretty fearless. The only time I see it uncurled is when she is fully relaxed when sitting or lying down, and even then, it doesn't happen often.