The power jack isn't really designed for much other than vertical loads to raise or lower the tongue... If you're going to be inside the trailer and moving, or working on it from outside, the leveling jacks should be lowerd to brace it in position. They will block any side loads, and absorb some of the fore/aft loads, along with wheel chocks as others noted...
It's pretty easy to bend or collapse power jack by pulling trailer forward with jack extended (don't ask..
) and it really is only designed for static vertical load.
John McG