Truck Tire Capacity

I to would be interested in what others consider a reasonable safety margin. In doing research on tires I spoke with the manager of a well known nationwide chain tire store. Although he would be happy to sell me a new set of tires and also would give me a trade in allowance on my existing OEM's, he stated that LT tires are down rated because as truck tires the manufacturer knows that they will be worked hard and likely overloaded. He recommended that even at full capacity I would be fine. LT tires used on DRW setups are down rated slightly because there are times when only one tire carries the full or the majority of the load. Not sure I'm on board with his "go to the limit" theory but it is interesting.