Soil? Hydro? Coco? Most of the time the seedlings have enough nutrition in the little rounded leaves (forgot what there called) for a week or 2, definitely should feed if the false leaves start to dry, but don't go crazy when they do or use it as an absolute guage. Give them at least a week before you start feeding extremely lightly. I personally try to keep the false leaves as long as I can, but I've been doing this for a few years. My self, i always go by the look of the plant. It's hard to explain but if you do it long enough, you'll get it.