Comment by colingauvin

Technically beta keratin is helical. That said you are correct, there is no requirement to be helical for fiber diffraction. It just turns out it's very difficult to have non helical uniaxial symmetry in nature.

When I was studying spindle shaped viruses by electron microscopy we'd get beautiful fiber diffraction patterns in the fourier transform of the micrographs. I always wanted to try and solve the helical parameters but never got around to it.