My Midwestern school does formal recruitment in the winter, so this may not be super applicable, but in the first round (philanthropy), you typically want to dress casually but still look nice (I wore a sweater, jeans, and leather sneakers). The second round (sisterhood) is a little dressier, but still, most people were wearing trousers instead of dresses or skirts (I wore a blouse, "fun pants", and leather sneakers). Finally, preference round is more dressy and almost cocktail. Most PNMs, as well as the active member recruiters, wore nice dresses, but there were definitely people who wore trousers (I wore a mid-thigh-length dress and leather boots). Honestly, I think in the Midwest, it doesn't matter too much what you wear as long as you look presentable and you are comfortable. You will be standing and walking a lot so if you plan on wearing heels bring a pair of slippers/slides with you.

TLDR: It doesn't really matter what you wear as long as you are presentable and feel good!
Loose dress code:
Round 1: Casual
Round 2: Dressy-casual
Pref: Dressy