I recently heard a back specialist say that doing that "tuck" disrupts the natural curvature of the lower back and spine and puts extra pressure on your discs. Yes, the ones which are prone to "slipping" (or, more strictly, splitting) - to quite agonising effect, as some people know.
Apparently we should be counteracting that ingrained unconscious habit by sticking our butts out backwards instead. Of course, we shouldn't take it to the opposite extreme (everything in moderation) - pushing your butt out too far or hard or holding it there rigidly wouldn't be very good for you either.
Hamstring stretches are also beneficial for the back because if your hamstrings are too tight, your lower back can't settle into that curve.
The aim seems to be to reach a balance, to help your back find its natural place again by letting your butt go backwards gently - which may initially feel like you're actively poking it out. It may seem weird and artificial at first, it may feel like tucking your butt under is in fact more "natural" just because we're so used to doing it, but with awareness and persistence that bad habit can be overcome. Hunching over our desks and computers isn't too great for our backs as it is, so we probably need all the back tips we can get.
One side effect to the butt going backwards more is that the the posture is less stooped - and the chest seems to stick out frontwards more too. So I imagine lots of people, not just you, would be quite happy with that, especially if you're female! See, it's a win win situation… good for you, good for others…
[Added 21 June:] A physiotherapist has since told me that, as you'd expect, it depends on the person; you need to find a position halfway in between your pelvis being tilted all the way forward (which is when you stick your butt out) and all the way back (when you tuck it under), and when you sit you may tend to do one more and so need to counteract it (by sticking your butt out) whereas when you stand you may need to do the opposite, it depends on your own habits - the key is to find the balanced, halfway position, whether you're standing or sitting.
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