Lookin great man, pullin up a chair to watch these babies grow.
For a first time grow, I reckon you're doin spot on, personally.
I don't see a massive amount of stretch anywhere, they all look pretty happy to me. But seeing as you're doin the pc-growbox thing, you will wanna get into a bit of training asap. As for how much to bend them, use your common sense, you've snapped a branch off a tree before, and you've bent one too... It's the same principle really, just go easy, don't go too extreme on anything that's already thickened up too much, but if she'll bend without too much pressure, then just keep her that way, over time you can make your plant pretty much go in any direction you want.
Good luck with the SCROG screen, I'm intersted to see how it goes, I ditched the idea and went with LST instead as my pc box is *tiny*, and it sounded like it'd make more sense for me.
Anyway, top work NH.