None of the stock trucks i've seen have had any heat sheild, nor have I seen any under bed wood damage as a result of heat.. but when I was putting the wood in my '46 1/2 ton the thought crossed my mind.
I wonder if a 6 exhaust gets all that hot in the first place? I know it's been 90 outside and i've run the crap out of both my trucks and while the exhaust pipe would definately be hot to the touch, I didn't feel any significant heat radiating from the pipe.
If you've got a fancy finish on your bed wood, tho.. i'd guess radiated heat could break the finish down over time, and eventually let moisture in.
All absolute speculation, tho.