Little or no preload means there is no bind or tension on the suspension links that would influence the corner weights. If you can slide the bolts in an out that easily, and/or if the adjuster is loose enough to wiggle by hand (no load on it), that's pretty close. Corner weights are a separate issue and are influenced by where the weight (fuel tank, batteries, driver, etc.) is distributed in the car, and spring loads. Neutral preload and equal weight on the rear corners is great. Should be easy to get it to go straight and square.

As far as having to pull the bars fore or aft to get the bolts in, the rear should be perpendicular to the centerline of the car. This is adjusted by running the front links in or out of the bar. If the rear is square, don't fool with them.