I've been playing w/ this lately, really trying to focus on getting my butt back and keep my shins as perpendicular as possible. I took some video the other day and I'm starting to rethink things.
My big problem is getting my hips back at the expense of losing a little back tightness. My arch is still there and my back is at least neutral, so not unsafe, but it feels mechanically disadvantaged. I find myself good morning the weight up a bit, or at least having to focus more on not caving in and following forward than if I didn't sit back as far as possible; when I hit the sticking point or the slowest part of the lift, I really feel a lot of force coming from my lower back, when I want to feel it more in my butt/hamstrings.
It also feels like I am going a lot deeper than I am. I'm just getting below parallel, but if I went any lower I would just cave in.
I need to get some video of the ones where I don't feel the extra lower back effort, and just feel the push from my butt and hamstrings. I'm assuming my knees will be a bit more forward and I may be able to get a little deeper.
I also squat in weightlifting shoes, so I will need to take more video and experiment more, perhaps w/ different shoes, but I think that I may be stronger w/o purely vertical shins.