Either positive or negative caster its main purpose is to keep to vehicle going straightforward down the road and help the steering wheel to return naturally to its center position. Manufactures tend to set a positive caster because thats been the theory for a long time. In my experience from what I've seen on import lowered cars caster is set between 6° and 8° looking from a vertical angel point. I don't now where the caster is on my fc but next time I get a chance at work ill measure it. Here's a google link to explain the effects of caster.http://www.ozebiz.com.au/racetech/theory/align.html

 

caster in itself has no self centering affect, caster trail does. but since trail is not independantly adjustable on jsut about any upright (probably none)... changing caster affects trail which in turn changes self centering.
the other big self centererer is KPI (SAI, steering axis inclination) which is increased by adding camber at the plates (thereby yielding higher angle between upper/lower mounts).

the real magic of caster is weight jacking (wedge) when you turn the wheel, which changes the under/oversteer balance of the car at different steering angles. a high caster car can be tuned to be oversteery at low speeds (high steer angle) and more stable at higher speeds (small steer angles).

generally speaking, more caster is better (within the limits of typical adjustments). if you have caster plates on your fc, throw that **** all the way back.

fyi

 

not sure if i'm late on this but when looking at coilovers for my brothers 240sx i talked to a friend about apexi and he recommended BC racing cause they were cheaper and of the same quality. crazy thing is most of the coilovers are made in the same facility. the only real difference is the materials used. But BC racing throws in quite a few extras which is nice. the guys over at citytireonline hooked it up.