2001 Ford F150 Supercrew. DRZ400S. Rough road with dips. Only tied down the front. Big dip, too fast. Truck goes down, back of bike stays up. Truck comes up other side of dip, bike meets tailgate. Overall structure of the tailgate is just fine. Inside had a recessed area where the bike tire landed. I got lucky.
That being said, I now tie the back down all the time. I've hauled my DR650 in it many times and my GSA once. Haven't killed the tailgate yet. The GSA was on pavement the whole time though.
I'd agree with the other comments. Depends on the truck and tailgate construction. But always tie down the back too.