Mulch and other protective coverings will conserve moisture and reduce the amount of watering needed. It also helps keep seed from freezing in colder weather, from washing away in heavy rains and from being eaten by birds.
Straw is an adequate, inexpensive mulch. Allow 1 bale for every 1,000 square feet. Be aware that any live straw seed will germinate with the new grass and straw also decays slowly.
Peat moss is a more expensive mulch, but offers some helpful benefits. It does not contain weed seed and easily decomposes into the soil.