What I have found, bearing in mind that I do not have the original jacks, but the higher capacity jacks installed, is I have near "O" flex and bounce.
My set up procedure is to level the TT front to back using my remote control tongue jack, then following up using the frame jacks to level side to side, starting in the middle on the low side.
I have found that the jacks tend to settle in the soft ground over time, requiring readjustment.
Maybe my expectations are different from others. I don't expect rock solid setup, it is a trailer after all.
I think that the fact that the center jacks are next to the spring hangers, they are lifting the trailer at their optimum location to both lift the trailer for tire changing, which I have had to do on 3 separate occasions, and for leveling w/o causing any more stress than the spring hangers. Then deploying the corner jacks helps to relieve
the bounce of the overhanging ends.
After all, the trailer is hanging off the spring hangers when parked and when traveling down the road.