The shortest answer is - it depends. I've seen different people use the term lst to mean drastically different things.
For myself, I aim to create the bend in the main stem (actually, more of an arc) above the 3rd node so I wait until the 5th node has begun (can just distinguish the leaves). Only the top inch or two of a stem is flexible enough to manipulate, GENTLY. If you look at my journal at week 3 you will see photos right after the first training wires are put in place.
The next couple of weeks are spent keeping the main stem as level as possible. As branches catch up, they are trained to the same height. The longest are guided to the edge of the pot so the shorter ones are left in the center. I can usually maintain a plant height of 3 to 4 inches to the edge of the pot, which is about when flowering is ready to take off, so I let them turn upwards then.
But this is MY definition of lst, others will have divergent processes with different results. What works for you is what's right for you. And experimentation is how we all learn things.
Good luck.