When it roots firmly. At that point, use of pre-emergents won't harm the sod, which isn't nearly as touchy as seed.

Sod laid in September, in Atlanta, of a warm-season grass, is probably pretty well rooted, but check it. And right now, it's dormant anyway, so if you were planning on striking it with a pre-emergent in, say, March, that shouldn't be a problem.

Jumping the gun won't kill the grass, just slow the rooting--pre-emergents root trim, but sod does already have roots to it. If you keep watering appropriately, the sod will live. The pre-emergent binds to the soil pretty high up.

With first-year sod, you'll need to keep an eye on it anyway. The root systems and general health just aren't where they're going to be in Year 3, so it's probably going to need some coddling to begin with.