I have a "Rock Guard" mud flap and used it with my previous hitch (travel trailer) as it secured to the hitch shaft. Then, when I changed weight distribution hitches, it no longer work, or can be attached now. I have mud flaps on each wheel well. Although I am towing a trailer and not a 5er, I really saw no difference with, or without, the Rock Guard. I think the mud flaps in the truck wheel wells do as good or more than anything. Regardless, with or without the Rock Guard, the trailer still got just as dirty from road grime on the front cap.
If you have mud flaps on your truck wheel wells, I won't mess with diamond plate either.
If you don't have mud flaps on your truck, get them. It's money better spent than on the gigantic brush that hangs from the bumper.