Tire pressure is not a set in stone forget it thing. High tire pressure equals lower rolling resistance, because on a smooth surface, hard tires flex less and create a smaller contact patch. But no road is perfectly smooth (we wish it was). Properly inflated bike tires conform to bumps and absorbs shocks. Over inflated bike tires has a harsh impact to the rider which can cause sore wrists, hands and arms which causes rider discomfort. Use the recommended air pressure that is marked on the side of your tires but play with higher and lower pressure until it's just right!
Once you become an expert at tire pressure you'll be able to cop a feel and know if your tires need air or not.
Having a Floor Pump with a gauge is the most accurate way to make sure your tires have the air pressure they need.