Depends on different things like weather, alignment, road conditions, ect... When I went to get mine aligned, the guy told me to keep the fronts at 50 PSI because I have a slight negative camber issue in the front... My tires are rated at 55 max PSI...

Heres a page which tells you a little bit about proper tire pressure...
http://www.tirerack.com/tires/tirete...e.jsp?techid=1