TheWakes.im 5'9" 140 and i ride the 178 dente. mounted at -1.5 from center. i wouldn't imagine wanting to go shorter unless u were doing exclusively small rail features. they are plenty soft and the rockered nose helps them feel shorter. they butter very easily.
if u want to jump at all on them go with the 178. especially if u might grow more.
if u want to do presses on small features and don't ski much all mountain or jumps u would have a lot of fun on the 168. just don't expect to ski fast and always be ok with this length. they are soft.
So i ski 164 j skis right now, I don't need them to be awesome out of park, just good enough to be able to turn. Is the 188 the right choice?