Within the the center buble is good.
If your leveling system has indicators, use a 24 inch contractor style level on the galley counter top to see how close the leveler indicators are. Adjust the levelers control so it is accurate.
Leveling blocks are not normally used in our case. We carry a set of blocks. We also carry four 12+ inch square plywood pads to go under the levelers. They are used in soft soil to spread out the weight.
I haven't used the plastic Legos in several trips. Considering leaving them at home. Then I will need them.
Definitely keep the rear wheels on the ground. My rig has parking brake on drive shaft. On uneven ground lifting too much weight will cause the rig to roll off the kick down jacks.