Fertilize 21-28 days after seeding.

With organics, it doesn't matter so much, but I'd wait a week or so after seeding. Since organics take about 3 weeks to kick in, that means the best availability will be about a month after you seeded, and continue onward through a good chunk of the rest of the year at a lower level.

Synthetics, 21-28 days like BeckyBeck says.

I try to avoid walking on baby grass. Maybe then it would be better to fertilize at seeding time, if using organics? You have to walk over the yard to fertilize. What do you think, Morpheus?

Sure, either way. Organics are extraordinarily forgiving.

If you can manage the hover walk, feeding on baby grass is no problem. I did it during my renovation as I was also transforming the soil at the same time--so every seven days during the reno, with KBG progressively sprouting, I was very carefully walking on it.

No problem. Just don't slide or anything and it'll be fine--sprouts are a lot more resilient than we give them credit for being (I do stuff in my flower flats with the sprouts that would curl most people's hair).