Typically, its the 5th node, however, each strain is different, you have to watch your plants, they will tell you when they are hungry. Now with that being said, you need to get those main stems a little more 'bulky' to support the plant. I see that you have a fan in there, but it looks like that the air movement is missing your plants and that is why you have to keep them posted up. First thing i would do is to get those stems thicker, and at the 4-5th node, or when your plants are starting to show signs of nute def, then you add the nutes. Sending a plant into a lockout at early age will result in hermie, male plants, and i dont think thats what your looking for.....i could be wrong tho.