I went to a job fair in FL and I was amazed at how underdressed some of the men were (and I'm a man). I thought suits were mandatory, but more than a few were hired wearing just a shirt and tie. I don't know what constitutes proper dress for a female, but I thought most of you would like hearing a different perspective.
I would strive for the most professional image I could project. If your slacks accomplish this goal, then they are fine. If you have a nice suit with a skirt, and you are questioning your attire, where the skirt.
I went to an interview myself the other day--my first in 12 years! I wore a skirt and tunic. I didn't have any pantyhose or shoes that look good in pantyhose. I really felt casual-ish for an interview, but I still got the job!