when you change the angle of the line between the front and rear suspension to the road (ie lower just the front end or lower the front more than the back etc) you change the characteristics of the suspension.
when you lower the front end of a beetle the caster angle is reduced which makes the car react quicker and more to movements in the steering or changes in the road.... great if you want to flick/slide through lots of tight corners but really dangerous if you want to drive at any medium to fast speed. (very unstable).
by adding castor shims you replace the castor lost when the front is lowered. this keeps the car as stable as it was.
on drag cars i think the trend is to increase the caster to more than stock to improve the high speed stabillity (they loose cornering abillity at the same time but there aren't too many corners on a drag strip).
talk to one of the bigger VW shops and they can give you advice on when to use shims and what thickness to add. if not try the cal-look forum
http://www.cal-look.com and go to the forum.
there are lots of really knowledgeable people there who can give you all the info you need to get it right.
hope this makes sense and is helpfull.
h.
[Edited on 9-4-2003 by whatnow]