On my '04 ford ranger, my Veestrom's rear wheel actually sits right at the leading edge of the tailgate when the front wheel is pushed all the way up to the front of the bed. Tied down 4 corners, pulling it forward/down in the front with ratchet straps and then back and down in the rear. Nothing under the rear wheel. Hauled it around 30 miles that way with no damage.
I used to haul my Ducati up to Maine and NH and such all the time back when I had that, but the point is moot there because there was no weight on the tailgate at all.
If I were hauling a longer bike, I'd probably put something under it to keep the weight off the tailgate... according to my owner's manual max weight rating for the tailgate down is 100 lbs.