It is only needed for new establishments or if you have a lot of bare spots, especially if they are on a slope.  The straw will raise the humidity around the seed increasing germination.  

For best practice, cultivate any bare areas first making sure the seed has a good soft bed to root into.  Spread your seed and scratch in with a rake for good seed to soil contact.  Use lesco starter fertilizer to add required nutrients to the soil which will also help feed your grass as it starts to germinate.  Then topdress with straw, lesco triple start, or peat moss.