The best way to decide when fertilizing grass seed is needed is to test your soil. You can buy a soil testing kit and do the test yourself or collect soil samples from around your yard and send them to be tested by an expert. Next you would look at whether you are starting a lawn from scratch with grass seed or just filling in bare patches.
If you are filling in patches then you will probably not need to worry about fertilizing grass seed as you apply it. Consider when the last time you fertilized your lawn was and go from there. If you have not yet fertilized it this year then apply fertilizer and lawn food at least a week before you consider spreading the grass seed. You want it to have time to absorb into the soil before you spread new grass seed over it. This is also true if you are starting a new lawn from seed. You need to let the fertilizer absorb into the soil before applying the grass seed.
Read more about fertilizing grass seed.