Truth is 350lb is with in the capacity rating to not need a WDH. You would only need a WDH once you get past 500lb if you have a 1500 series truck. Most 1500 series trucks have a 500lb weight carrying limit on the hitch. So once you get past 500lb of tongue weight, you are not only unloading the front axle, you are over stressing the hitch itself. at that point you need a wdh.
Of course the weight at which you need a WDH is based on the weight carrying capacity of your receiver hitch. 2500 series, and 3500 many have a higher weight carrying capacity.
My F150 has a weight carrying capacity of 500lb on the receiver, and a WDH tongue weight capacity of 1050lb.