If yours has a leaf spring axle
, as some do, under the axle is okay. If it has a rubber torsion axle, as many do, it's better not to, although done gently on the outer end near the mounting brackets is unlikely to cause harm.
Best practice, though, is to jack on the main longitudinal frame rails just aft of the axle mounts. That part of the frame is designed to carry the full weight
of the trailer when the wheels are on the ground.
Lug nuts on backward... very bad.