The way my fitter explained it when I went to get fitted for a custom crank set for my LLD, is that things like saddle height, fore/aft position, crank length, cleat positioning, are very scientific/measurement based, but reach is much more comfort based. Yes, the more aero your body is positioned the faster you can go in a wind tunnel, howerver, if your body can't hold that position comfortably it gets you no where. So, while my fitter set up all the scientific things for me (including designing a custom crank set with a dropped pedal that corrects a 3 cm LLD), he left the reach part up to me (of course a fitter can recommend a starting point, but you need to tweak it). He recommends riding with a fit stem to find the right balance between aerodynamics and comfort for your body.