I have kept my Lance 981 on the jacks for nine years with no difficulty. I do, however, lower the unit until the rear bumper is only about six inches from the ground. I keep the front a little higher.
On a related note, I sometimes see campers off of the trucks in campgrounds with the jacks fully extended. In my opinion, one should always lower the unit almost to the ground for stability, safety and just easier to climb into.
I also have often seen people camping with the jacks extended down to the ground while on the truck. I don't know why they do this, mine doesn't rock very much no what I am doing inside and the risk of forgetting and driving off thereby destroying the camper is present.