Water is essential to all life....too little water and we die, too much and we drown. The same is true for the grass in our lawns. The fact is, more lawns are damaged or destroyed by over-watering than by not watering enough. Proper watering from the beginning will have an impact on how well the lawn continues to flourish for years to come.
That said - it is best not to water your lawn when the rain is coming down like cats and dogs!