Weight in the bed DOES make a small difference, but not enough to offset the importance of having weight properly distributed on the trailer. Whatever weight you have in the truck behind the rear axle IS going to add to the weight that a WD hitch has to redistribute, but it does next to nothing to resist side-to-side twisting (the "yaw" Ron mentioned).
Oh sure, it will add a little to the inertia of the tow vehicle and it may make it a little easier to maintain weight on the front axle, but inertia is of little help in resisting side-to-side twisting compared to having properly weighted tires on the ground and a trailer that isn't tail-heavy.
