Use a reliable thermometer to make sure the plants are under 80ºF and the soil should be less than that.
As far as root showing, as soon as the seed cracks is good. It doesn't help to let it get any longer. Many of the better growers put seed right in the medium. I put them in a damp paper towel in a baggie in a warm place (not over 80ºF) and 24 or so hours later they should be starting to crack. That's the time to put 'em in the medium. I use T5HO lamps about 3-6" over and I run a fan across the light/plants. My plant temp is about 70º this time of year. They are growing very well. Don't put them too deep, just barely cover the seed with medium.