IMO, it is better to add weight to even things out, than using your spring collars (preload). If the chassis is tweak free, the shocks are the same length, and nothing is bent/binding, then your spring collar positions should always be equal.
When rebuilding the car (including shocks), I always put the collars at the top. Then with each front/rear pair, I put a small mark (sharpie) in the same position on the collars. Then I use the mark to make sure I turn the collars an equal amount, until I get the desired ride height at each end.
It shouldn't matter too much if the ride height difference from left to right is less than 1/4mm. Unless your racing in the A main at a regional/national event.
