I usually stop popping once they're onto fuzzies, if they're that big then the males are strong enough to hold in the hemipenes and easy to get "female". However, I don't feel comfortable probing at that size, I usually won't probe until they're to hopper/small mice.
There's a window that I personally don't feel comfortable sexing a cornsnake. They're either too big or too small.
As long as you know what you are doing, it is safe to probe them right after they hatch. I ususally pop my hatchlings first and then probe them if nothing pops out, just to be certain.
Reptile Collection: Amazon Tree Boas, Ball Pythons, Boa Constrictors, Brazilian Rainbow Boas, Carpet Pythons, Chondro, Corn Snakes, King Snakes, Milk Snakes and a Retic. Too many morphs to list anymore!