No, the harder rubber in the tires when they're cold will still flex - if you pump them up to 100 psi and load each tire with 100 lbs., then the tire will flex until you get a contact area of about 1 sq. in. But when the tire is cold it'll take more energy to do that flexing and therefore the rolling resistance is increased. You want tires with very supple, easily flexed sidewalls for minimum rolling resistance.