If you have done everything to prepare the soil for your new lawn, then fertilization will give it that little extra boost.   However, if you are just planting grass seed without preparing the soil at all, fertilization is not necessary because it won't help anything! It might actually hurt your efforts by burning out seeds before they sprout.    There's also a chance of fertilizer working its way into streams and waterways where it can harm aquatic life.   Remember - before using any kind of non-organic fertilizer always check with local regulatory agencies to make sure you're not breaking any laws.