You should inlfate them typically to what the manufacturer recommends. The engineers selacted that pressrue to match the suspension components/set-up and strike a compromise between ride, handling and economy. The tire pressure is often dependent on vehicle weight. So a 2500lb car with the same tire width as a 3500lb car will have often a lower pressure recomendation.
That being said, I sometimes increase my pressure 1-2 psi on long freeway trips ot improve mileage and reduce wear. I will also increasde my rear tire pressure 1 psi to better balance the handling of the car. Most cars on the road are set-up to understeer to be more user friendly for the common unskilled driver.... so they are less likely to lose control.
Most of all inflating them too high really makes the car ride rough on most roads. and on rough roads, you will actually get lower mileage and increase tire wear because the tire will have a greater tendency to lose contact with the pavement.