*ignorance speaks here:*
Would it vary depending on what grass you're seeding? I always thought protecting grass seed from birds was a bigger issue than time of year. I'm pretty sure I remember seeding a small area with rye grass seed in September and having it do just fine, but that was in Siskiyou County, CA (very different climate) and it was on top of where I had removed a compost pile so the soil was killer-rich.