a over-inflation (a bit) is better in almost every way to under-inflation
simple demonstration
Tire Tech Information - Air Pressure vs. Wet Performance
but I do agree with Neil - each tire is different
the autocross guys over-inflate not under-inflate and they certainly would not accept a performance loss.
if you are running 32psi because your door placard tells you and your tire says more, you are probably under inflated and that IS dangerous.
if you run 32psi because your tire side wall says so - you are on the verge of under-inflation the moment it gets cold or you have any kind of leak, and that could be dangerous.
if you are 5 psi over the side wall - that has got to be minimally significant for safety or we would have people killing themselves because of low accuracy tire pressure gages. If you really want to pop a modern tire (with out help of a nail etc.), you have to run it low.
60 psi in a 32 psi tire - well... that is an optimization issue as stated
personal data:
bought truck and started running 55 psi in 44 psi rated tire on an S10
wear pattern improved - handling improved - rolling resistance improved
ride rougher and road noise higher pitched and more noticeable
wet traction was poor before and I have not notice a change - hydroplaning reduced