From what I have experienced is it also depends on how much side to side movement your hitch head allows you. I had a Reese 15K which was quite limited when it came to side to side motion and when the dealership set up the trailer, it only had about 4 inches of clearance on the rails. Never hit the bed rails because the motion was limited. I purchased a new truck and hitch, (Curt A16) and was very surprised at all the side to side movement this new hitch has and set up the clearance to almost 6 inches for the bed rails.
Curly