Tilling a yard used to be an accepted normal practice. You would make a few passes through grass and weeds and are left with "fresh, healthy soil" to start over with. This actually isn't the case. 

Tilling for the sake of tilling can lead to big problems. It can damage soil structure and spread perennial weeds if they are not eradicate before hand. If you want to till your yard, it should be to add a soil amendment that you want to get to the root zone quickly. After tilling, be sure to roll the lawn with a steel or plastic drum roller to ensure a uniform grade. This will also help reduce airpockets and ensure contact between seed and soil if you have overseeded.