If you decide to jack up the car for installation... it will relieve the stress on the frame and then when you lower it again with the bar; the bar will be pre-compressed which will make your car even stiffer. That's what I did when I installed mine.
Note that for this to work you have to jack up the car from the centerline and this can be quite unstable. If you decide to not jack up the car from the centerline and use the regular lifting points on the sides you might as well not lift it up at all since the frame will still be loaded.
Hope this helps.