The max tire pressure should be on the side of the tires. It's usually 44psi. Lower profile tires require higher pressures to stay responive and keep tire wear even. If you want good gas mileage and tire response keep it higher around 35-38psi. If you want a soft cushy ride trade someone with 15's and tall tires and lower it to 28psi. I personally run 40psi on all tires. The ride is firm but that's how I like it.
The correct pressure is what will allow them to wear most evenly.
Meaning... start with something like 35psi and see how they feel. Check the tread often and see how they are wearing.
If the outer 2 edges are wearing more than the middle, they're underinflated... RAISE PRESSURE.
If the middle is wearing more than the outer 2 edges, they're overinflated...LOWER PRESSURE.
With lower profile tires, you'll want to run higher pressures to help keep from slamming the rims on bumps, but too much will make the ride harsh and wear the center.
Higher pressures will make them work better in the rain due to less deformation of the tread=better water evacuation.
Higher pressures will also get you better gas mileage, if only marginally.
Best thing is to check them often and adjust as needed.
And remember that you lose pressure as the temps go down.
I have 225/45/17 and I run about 32psi and my wear is even.