Dan, in my opinion, there's no one right answer to your question. Or the right answer is "It depends." When your spine is vertical, shear forces are minimized, and less than a perfectly straight spine is much less likely to cause injury.
Personally, I love to squat light and super deep, which means my back must round a little. When it's bodyweight only, I manage to round in my upper back a little instead of my lumbar and I think that's safer. But when I go heavy in the barbell front squat - NB: my heavy isn't even Dan's warmup - I need to sacrifice some depth in order to stay tight at the bottom of the lift. And that brings up another important point - some relaxation is necessary for maximum depth in the squat, and you don't want that with a heavy weight.
A goblet squat, because it's such a light weight, is a movement I treat like a bodyweight squat and I think very deep is good there. A double kettlebell front squat, on the other hand, can start to be heavy, so I don't recommend going super deep if, e.g., you're front squatting a pair of bells that's approaching your bodyweight - and for someone new to squatting, I'd insist on a straight spine at the bottom of even a light, one kettlebell, rack position squat.
I think one of the best teaching explanations I've seen of the principles involved here is how Marty Gallagher teaches. He'll have you go rock bottom with a light weight, even purposely exhale and completely relax before retensioning and coming back up, but as the weight gets heavier, he teaches you to stay tight.
Tail tuck, we should say, can be an issue not directly related to squat form or depth - it can indicate poor flexibility in the hamstrings, hips, ankles, or t-spine (and I'm probably leaving out a few). This is why it's good to be able to move well and freely into a fairly deep bodyweight squat with no tail tuck before attempting to load it with weight.
Just my opinion, your mileage may vary, and I hope that's helpful.
-S-