The plants will go through a transition from veg to flower that I don't even count towards the "flower period" given by the breeders. I count Day 1 of flower as being the first day I see pistils, and then start counting from there. And that's often 10 days or more after I flip the lights to 12/12 (actually in my case I move them from a veg box up to a flower box, but same idea).
Right after they stop stretching there is a short window that you can use to do the defoliating without worrying about causing too much stress and therefore hermies.
But do what you're going to do right then and then no more, because you're right, stress too far into the flowering period can cause issues like hermies. And make sure you have no light leaks in your tent from seams, or the indicator lights on any timers or other equipment in the tent. Any light seen by the plant during the dark phase can cause issues.
Later on, we're going to suggest you consider draughting them to increase the trichome production, but that comes very late in flower (week 7 of a 9-10 week strain), so even if it does throw male flowers that late in its life, any female flowers that might have been pollinated won't have enough time to do much damage. BUT, the tradeoff is you get a dramatic increase in trichome production.
But, one thing at a time. Clean them up a bit after stretch and then baby them for most of the rest of the flower period, and we'll circle back to the whole draughting thing as you get closer.
