Caster is responsible for the steering to naturally go from turned to straight (if you let go of the steering wheel).
You do want positive caster for this to happen. I run as much as I can get on my track car, hopefully about 5 degrees.
Positive caster is like a the front wheel on a grocery cart. The front wheels want to naturally go straight. (Stuart, I think your example of a long-forked chopper is actually negative caster.)
