On a mountain bike, crank length doesn't make a big difference as you're not consistently pedalling with your body and legs in a fixed position, you're always moving around, backwards and forwards, weight off the saddle, then sat down, freewheeling, backpedalling etc. over terrain. On a road bike or bikes that you only use on the road over longer distances, it matters more. In this case, your legs are spinning over and over thousands of times in the same position, so if you are over stretching then you'll start feeling it eventually.
Really doesn't matter for an MTB.