Start em at 30 psi and move up/down from there.
Over inflated tires weaken the tires contact patch on the road, tire wear is increased (right down the middle) and safety is compromised. And they ride like crap.
Under inflated tires have the same effects on contact patch and safety but they wear on the outer edges, over heat the sidewall, and ride too soft on the road, steering response and cornering ability is decreased.
I'd be more concerned with safety and long life on the tires than I would be concerned with rolling resistance (fuel mileage). My life is worth alot more than a few MPGs. Plus, I can't afford to replace tires every 10-15k miles...