I quite like when the WG looks a bit dishevelled from her exertions at the end of the punt, long hair naturally helps create that look!
See my recent review of Ling (Cosmopolitan, Manchester). She has long hair, and it was really striking for me how thick the strands of her hair are, which I gather is the norm for orientals. But it was a new sensation to my touch to feel her thick hair, totally unlike the hair of any western lady.
But I do also agree with enjoying watching the lady suck me, especially in a mirror. So long hair down, but with the option to get it out the way to improve visibility would be my preference - all else being equal.