Originally Posted by
CristianTabush
It is important so that at ride height, your pistons sit at the same position in all 4 corners. The piston height has a bit of an effect on how the oil flows through. Having all shocks the same length ensures all 4 shocks are "equal" which should make the car more consistent.
This also allows you to set your shock collars at the same height L/R and can help you identify when your car might have some tweak.
Total shock length will have no affect on the shock collar height.
Yes, it will change piston location. But even without calipers, you wouldn't be out by more than 0.5mm as long as you use the threads as reference. I'd be surprised if 0.5 difference in piston height had a noticeable effect.