Does anybody here believe that 5,000 or 10,000 pound tow ratings are the result of an exact mathematical calculation? These are obviously general guidelines with a built in fudge factor. Otherwise, slight variations during the manufacturing process would result in some failures even when below the "maximum tow rating."
I am not suggesting that these rating be ignored, only that they be taken with a grain of salt when evaluating the towing capability of your rig. For example, dead tongue weight, weight distributing and four-down towing hitches are drastically different towing propositions, as are terrain, elevation, braking capacity and driver experience.
That being said, I would not greatly exceed the specified ratings for fear of voiding warranty and/or insurance coverage.