Correct, you don't need it on a site that is leveled.
A friend of mine was unhitching his trailer in the storage place. As the trailer disconnected from the ball it moved towards the car in a slight angle. Luckily the stinger stopped the trailer with no damage. He then used the stabilizers to get it off the car and fix the other chocks below the tongue jack (they tilted and gave way).
I was there and it was scary. We could have hurt ourselves or damaged the car. After that I will always chock the wheels. I even do it when I just stop for something since I don't trust my car emergency brake.