I wiped out once doing that. I was riding an XL250 up a 6" ramp, and went too fast. I got on the brakes once in the truck, but wet tires and bedliners don't go well together. Ended up laying the bike down, with my shin on the sharp peg, giving me a 2" gash to the bone. I still have a nice scar to prove it.
The best one I watched was a seasoned logger with a chain saw strapped to the back of his XR600. Just as I slowed to ask if he needed help, he gives the bike a little throttle, and it coasted nicely up the narrow ramp as he walked beside the truck with one hand on the seat, the bike stalling just as it came to the front of the truck bed. He looked at me like "what would I need help for?" You could tell this was his daily routine.