Angling the ball toward the trailer will put the chain ends of the bars nearer the ground. Thus you will pick them up further to attach the chains and achieve the desired "bars parallel to the frame" situation. This will shift more of the tongue weight to the front axle of the truck.
Measure the distance from the wheel opening in the fenders to ground on the truck before attaching the trailer. Then measure again after connecting the trailer and weight distribution bars. The truck should squat nearly equally at the front and rear fenders if set up correctly. If the rear squats a lot more than the front, the truck/trailer combination will "porpoise" or buck when towing. You can tell when the towing ride is acceptable.