kys,
i dont think originally there was anything between the bumper mount and the body, but i added some rubber washers (lowes), seemed like a good idea. also, be sure you have a buddy over when you install the bumpers, easy to scratch the paint if solo.
not sure what your plan is for bodywork, but make sure you plug all the holes, especially the ones from that nasty skirt piece on the rear that is attached in about 20 places. until i repaint my car i got some black plastic plugs (lowes again) and rtv'd them in place - looks ok for now and def waterproof. pic below.
happy to help with more specifics if you need, pm me if so.
best,
alanmcg