Mulching over freshly planted grass seed with a light blanket of straw or hay can be beneficial for a bevy of reasons. Straw can hold the seed in place in windy climes and prevent birds from making off with the seed. It can also keep needed moisture in the soil to aid in germination and prevent opportunistic weed seeds from landing on the richly prepared soil of the seed bed.
Purchase enough clean straw or hay to cover the acreage of your grass seed bed with a lightly scattered layer. You will want enough to create an airy grid over the seeded soil but still allow some sunlight to penetrate and fresh air to circulate through the straw. A rule of thumb guide for the amount of straw to apply is one standard bale to every 500 square feet of seed bed.
Hand cast the straw over the entire seed bed treading over the soil as little as possible. Throw handfuls of straw over the area to give an even blanket effect including coverage of the edges of the planting bed where seed can most easily be displaced. Make two passes over the area as you cast the straw with the second pass walked at a 90-degree angle to the first pass to ensure even coverage.
Lift half of the straw from the seed bed when the green grass blades reach roughly 2 inches in height. This can be raked up very gently with a flexible tine rake or by hand. Be careful not to rip or pull the young grass blades. Leave the remaining straw in place to act as a nutrient mulch on the grass. Over time, the straw will degrade and become invisible blending into the soil. Refrain from mowing your new lawn grass until the green blades reach at least 2 1/2-inches in height to prevent stress on the roots and damage to the tender blades.