When you go over a bump (like going over a RR for instance) that puts the truck on a different plane than the 5er you may need every inch. A long bed will need more clearance over the same bump than a short bed. 6 inches may generally be enough but it may not always be. Just keep it in mind as you go through washouts, over RR grades, over speed bumps, etc. You can always back up Or just do not worry about a dent or two in the bottom of the overhang--it does not hurt anything and most folks cannot see it anyway :)
BTW a dent puller does a pretty good job of fixing them,