I'm using a normal rubber mat that was made for my truck, and I wouldn't haul the camper without it. Mostly because on my truck, the tailgate is proud of the bed floor by about the thickness of the mat, so it evens out the bed floor with the tailgate, and keeps some of the weight of my camper off the gate. The tailgate on my truck is line-x'd, and the screws on the bottom of the camper have dug into the liner on the gate and made nice holes, so even with the ~3/8" thick mat, there's still significant weight on the tailgate.
I can't see how using a rubber mat of some kind could be bad... It certainly keeps the camper from shifting as much as it would on painted steel. I think LineX would be fine too, as it's somewhat grippy. IMO, tie downs do not really stabilize the camper that much from side to side, and in my case, the front and rear tie downs both pull slightly forward, so the bed mat helps keep it from shifting... It'll still scoot up against the front of the bed if I do rougher roads, but once it does, it doesn't go anywhere.
YMMV.
C