I'm pretty sure that a sprout can see the light when it is 1/4 or 1/2 inch below the soil. That little bit of soil is not solid, and light does penetrate the surface. Also, when I close my eyes I can still tell a difference between light and dark. I'm fairly certain that plants don't have eyeballs, but just to be safe I try to get a little light over them at least within 2 days of planting. Helps keep em warm too. I imagine you would not really need to do so until you see the first sign of life.