When plants are first coming up, I wait until the 3rd or 4th node and top there. I have also done low stress training which is bending the plants after the two new shoots have developed, then I tie those down. You can do this later, but you have to bend where the plant is still supple, so it just depends on your plants and how long they have been growing. Mine were done after approx. 2 weeks old. Here is my result to this plant.