Tire Inflation: Proper PSI to run?

[ty at festf.org (Tyler P.)]

Ok, how do I know what PSI to run on a car?  Should I go by what the max
psi rating is on the sidewall of the tire?  Somebody on vortex said to
always go by what the car manual says but this doesn't make sense if I'm
running after market wheels and tires.  Once, at discount tire the guy
told me that the kuhmo 712's on my scirocco had a lower load capacity
than stock so they should be running a higher psi, also the goodyears
gsd3's on my bmw says they have a max psi of 51.  I've always just run
around 32psi but should I be doing something different to determine the
proper psi?