I do a little of both. I only had an issue riding up once, on a DR650 with mud covered tires. Back wheel slid of the side halfway up. If tires and ramp are both dry, and the ramp is already in place when I pull up to the truck, I ride it up. If I have to get off the bike to get the ramp out, I push it up. At my house I can get the tailgate less than a foot off the ground. I just bounce the front wheel up with the dirt bike, and use 1/2 a folding ramp for the street bike.
I do have 2 fail stories, in addition to my getoff -
1 - My brother rode his V-Strom into the back of his F150 for several years (not that often though). First time into his new Tundra, he didn't get it stopped quick enough and broke the back window (which is one of the fancy roll-down ones).
2 - When I was younger my father used to stop at the car wash on the way home from dirt riding. We would jacknife the trailer, unload the one from the bed of the truck, then wash the bikes on the trailer, then the unloaded bike, then jacknife the trailer again and load up. He rode it up one and only one time at the carwash. Wet wooden ramp + soapy wet tires are not a good combo.
When pushing the bike up (or unloading), I put it in gear, engine off. Release the clutch halfway up and then move into the bed (or down to the ground), pull the clutch back in, and finish. No step needed. Then again, I'm kind of a big boy, so balancing a Kawasaki Versys with one hand while climbing into a truck isn't that big of a deal. If you only weigh 150lbs, it might not be quite as easy.