Preload is one of those adjustable factors, so you can adjust it; the question is: do you want to? Coilovers are generally made with pretty close to the right spring rate for the dampers without any significant additional preload. You can add more if you want to firm up the handling (and the ride at the same time).
The way they are usually set up from the factory and the way that I've read most folks here recommend that they be set up is to loosen the collar until you can move the spring up and down just a bit, then crank it back down a few turns. You should still be able to rotate the spring by hand, but there should be no up and down or free loose motion by the spring. (If your suspension is very noisy when driving, you're still a little too loose.)
This is essentially, no preload or minimum preload. This is at least where you should start with your coilover setup. If you still want a firmer ride and handling response, then add some additional preload. Note that as little as 5mm or less in spring preload compression is a *significant* increase in overall suspension stiffness. If you adjust preload, you will only need to do it a few collar turns/mm at a time.
In terms of preload adjustment affecting actual ride height, there is some minimal effect on ride height by making the springs stiffer before the weight of the car is resting on them ... the car won't far as fall once sitting back on the suspension since you've effectively increased the spring rate by adding preload, but it is relatively minimal compared to the actual ride-height adjustment. You really want to set the ride height by determining how far up the lower suspension mount is threaded on to the center threaded strut section.
This is of course extremely easy to change before the coilovers are mounted on the car. It's a little harder once they are installed. You gotta think about which collars are turning which way to get the center threaded part to spin the right way to go up or down as you want since the lower strut mount is fixed in place (rotationally anyway) once the lower strut bolt is installed. Still beats pulling the lower strut bolt and reinstalling it just to change ride height.
Last edited by Dujo; 07-19-2012 at 10:28 PM..