Look at this way ... being up on the QJ for months my help to un-do the decades of the weight of the car resting on the front wheels and the frame sagging the other way
I have my front blocks pretty much right on the heavily reinforced front "turn-in" on the frame, don't see any sagging when it goes up.
A car frame should flex without issues, just because it flexes doesn't mean it's bending. Takes many, many years for something like a frame to take a "set" .... in this case any set would go away the minute the car was lowered to the ground.