I don't have a recommendation for your case, but I'm in a similar situation, although my now 13yo daughter is just under 5' and weighs about 105lbs. We also live in the East coast.
Based on online reviews.I got her a pair of Roxy Broomstix (www.evogear.com/outlet/skis/roxy-broomstix-womens-2009.aspx
) (122-86-110) 158cm (just over her head) mainly for skiing powder, crud and trees this year. At the time, I also was looking at the Salomon Suspect (amongst others), which was unisex, more expensive and was only available in 162cm in that range. She probably could go with the extra 4cm as her previous twins in the last two seasons measured exactly her height. Because here in the East our powder days are far fewer and the off-piste turns are much tighter.
In a couple of month, we'll be able to found out whether I made a good call. Even though I, later on after the purchase, found out that the Suspect could a bit more agressive of a ski more suitable for her style.
As for your daughter, I think around 172cm should be fine for a twins or AM. If you were in the East, I probably would go with <90 waist for the reasons others have mentioned. Assuming she is as able a freeskier as a racer, I think she should do well with just a narrow-mid fat in all conditions including deep pow. Afterall, I suspect that she'll spend far more time on course than off. But, then again, you guys have a much longer season than we do.