Depends on who you ask. Apparently, studies have been made by law enforcement agencies that indicate you should run the maximum sidewall pressure for best efficiency and performance. (of course, those of us who know more about handling performance know that isn't necessarily so, but it's probably better than running the factory pressures) Hypermilers will tell you to run more than that, up to 50 or 60 psi, and that they've been doing it for years without issue. And, of course, the vehicle manufacturer is going to tell you something completely different.

If you're really out for max fuel economy, I wouldn't hesitate to run max sidewall pressure and just watch for any signs of excess wear in the center of the tread. Much depends on the tire... and on the vehicle.