I bought my first road bike, a one and a half year-old focus izalco pro, this summer. It came with conti grand prix 24's on it. I'm not certain how to tell when the tires are worn out. I come from a mountain biking background and it's fairly easy to tell when a mountain bike tire needs replacing. There are a few little holes on the tread of the tire that look like wear indicators,but I'm not sure. The rear tire also has a smaller flat patch down the middle of the tire, where the front tire is more rounded. I didn't pay attention when I bought the bike, so I'm not sure if this is normal or not. When should I replace the tires, when they get the flat spot or when the wear indicators are completely gone? Or is there some other method that better hes gauge when to change them?

Also, when changing tires, do you always put In fresh tubes, or just ride the old ones until they stop holding air?