Since the car is lowered, the angle of the trailing arm to the body mount will change. The 8mm stock offset will now be less since the body is closer to the ground but the center of the wheel didnt move. Either put your car together with the new bushings preloading it by eye to get it close (since your going to take it all apart again in a minute), then with all the weight on the car, measure and record the new angle that the trailing arm is situated at. Then take it apart, and install the bracket onto the bushing so that its the same angle it was when the car was put together with all the weight on it. That will cause zero preload on your lowered car. You could always measure another car that has the same springs allready installed instead of putting it together and taking it apart again.
Will it make your RTABS last longer, sure, will your really notice, probably not. These cars are about 10 years old and some driven hard. The bodies have shifted/settled due to their unibody construction, and its a rubber bushing, if your off by a few degrees, youll be fine, its called manufacturing tolerance.
As im sure you know, get it aligned after at that place up in Valencia and youll be good.
Its a car, not the space shuttle.
Huntington Beach, CA