Tires definitely change throughout their lifespan, but should perform as designed once the mold release is scrubbed off. One thing you'll find within the first couple hundred miles is that the carcass will stretch a little, so the tires "grow" somewhat. Something to keep in mind if your tires barely fit the brakes and frame after installing.
After that, the rolling resistance actually goes down as the tread thins and becomes more flexible. Downside is that the risk of punctures goes up, so it's up to you when to retire (hee-hee) them.