ZAM,
When your installing them with them with the cars tires on the ground your putting weight (you) in the trunk, loading up the rear suspension while installing the braces.
The instructions SPECIFY for you to install them using 4 jack stands, supporting the car BY THE FRAME!
With THAT said, I'm not sure how much difference REALLY makes, but think about this, how often do you drive around with 200 lbs in the trunk? These cars ARE NOT rocke solid, they flex, ALOT. Adding weight in one corner of the car will cause the body flex, and when you install the braces with the car on the ground that's exactly what you do. You put a little stress here, a little there, and then you have a brace kit that is installed with a little flex PRE loaded into the body.
ANY problem in your suspension will make installing it RIGHT impossible. From worn shocks, to bad springs, to bad bushings. Get the car OFF the ground, tires dangling, and install them it the ONLY right way to do it.
Ryan