I've always went by, Load it the way you are going camping and adjust the WDH to meet your situation. If you measure your truck (either with fender height or at the scales), what is acceptable drop in the rear and what do you want returned to your front axle? Perhaps half of the ATV weight is behind the truck axle and most of it is very close or in front of the truck rear axle. Pay attention to how much weight or fender height you return to front of truck.. 
My dad had ATV in back of his truck on his last setup (has toyhauler now) and he set WDH up so it was the fender height he wanted with his ATV loaded.
Your choice, likely not going to make large difference.. but on 1500, it may.