I think you can LST at anytime, just want to take care not to snap the stem when they are that little. I'm generally LSTing clones, so I do it when there are 3-4 sets of opposing branches. I've got a couple now that are two weeks in soil and all roughly 4-6" above soil, spread out about a foot wide now. Hopefully will go under scrog in a couple of weeks and I won't have to pin them so much.
So, it also wouldn't hurt them to let them go a little longer and get some strength in the main stem, you can bend a roughly 1/8" stem about 80 degrees as long as you aren't brutal about it. A U made from a coat hanger and sunk into the soil will keep it in place. Bonsai wire also works well, if you have it.