Have you heard the one about the spineless ski instructor? Oops I degress.
This is a great question.
A piece of advice that has served me well is "expose your belly button to the wind". I believe that would fall under the "IN" answer to your question.
In addition to "functional" stances, we also talk about "tall stances". When you see someone bent over at the waist (tailbone pointing behind, spine angled way forward), it's pretty obvious that they are not going to be skiing very efficiently. But if you were to see someone skiing with a perfectly vertical spine (not necessarily in the back seat), you'd probably be asking yourself "what's wrong with this picture" because it would look odd and lead to some weird skiing. I have a good friend named Tony. Tony knows how to ski (i.e. Toe/Knee/Nose in vertical alignment = how to ski). As a reference point, the spine should be angled slight forward to achieve this. But as JASP has noted, a reference point does not mean a static position to be kept at all times. Also, you can't move your hips (great point Zen) and keep your spine perfectly vertical and keep your upper body at the same height off the snow (part of quiet upper body). If you move your hips while maintaining a quiet upper body, your lower spine will move a little. That's ok. Rounding your shoulders (bending the upper part of the spin slightly forward) is something that was a defining characteristic of a difference between Canadian and American skiing that was most evident in the racers. I'm seeing less shoulder rounding from north of the border these days. To me, rounded shoulders does not appear to impact performance a lot so you can take it or leave it.
I will add one caution here that I learned from golf: rotating the upper body around a bent lower spine (e.g. bending from the waist instead of the hips) is a sure way to ruin your back. The idea here is to bend from the hips instead of the waist (belt line). In skiing we don't rotate nearly as violently as we do in golf. And we mostly want the legs to turn underneath the body versus the upper body to rotate around the lower body. But we do have some turning of the upper body and we can develop forces in skiing (e.g. momentum) that are far greater than golf. I think everyone can benefit from learning the difference between bending at the waist vs bending from the hips. But only those with healthy backs should dare experiment with the difference between rotating the upper body around a spine bent at the waist vs bent at the hips (and even then only very slowly and very briefly). I'm not saying "don't bend your lower spine". I'm just saying "be careful".
Expose your belly button and remember what Tony Knows and you'll be skiing like a champ!