I use both.. I level the truck and camper with wood board sections (I'm too cheap for commercial lego's) and drop the jacks for stability (I don't like a rocking camper, having spent the better part of 20 years sleeping in a big truck bunk that always wiggles around....
If the spot is fairly level (according to my torpedo carpenters level), I'll use the jacks only (so long as I don't have to lift the camper more than the suspension travel of the loaded truck because I want the camper in contact with the bed at all times. If it's out of level a bit, I use my wood board/blocks and then set down the jacks for stability. Either way, I always release my tie down's.