I've got tens of thousands of miles on my truck with a snowmobile in the back. It's fine. If you're really concerned about it, yeah, a sheet of plywood will help. With one bike, you can load it in diagonally. My bike fits in the shortbed diagonally, along with my 8-foot snowmobile ramp, with enough room to close the tailgate.
Depends on the truck though. I've seen some trucks (mostly the smaller ranger/tacoma types) that bend because of weight on the tailgate. I'm sorry, but if it can't handle two or three grown men sitting on the tailgate, it's not a very useful tailgate. And with that kind of weight on it, a bike or sled is not going to be a problem. Just keep an eye on the cables, replace those every year or two.