Sir, you are the only one who knows how and why the bushing must be installed properly. As you say, "You want the bushing to have zero stress on it at rest." You are 100% correct. Let me just add, "at rest" means the car is sitting on the ground and the suspension settled to its normal ride height.With RTABs (rear trailing arm bushings--NOT front control arm bushings), you do need proper preload (aka no preload) on the bushing because the bushing carrier rotates on the rear trailing arm. If you bolt it at an angle, then when you drop the car back down, the bushing will have a pre-loaded amount of flex on the bushing. Meaning it will be stressed at rest. You want the bushing to have zero stress on it at rest. I'm sure not even the factory always gets this 100% correct, but you can do your best to minimize flex. The 8mm below hub center line/straight edge rule was developed as an alternative to the expensive BMW tool.