I think they want to know how committed you are because the effects of HRT are not reversible. Don't sweat wearing dresses. I hate them to! My last visit to the endo she had a Dartmouth med student in there with us. She figures its good to educate them on trans issues, Which I don't mind, I do it just about every time I go. But I basically tell them to ask any questions they want. Well one was so do you wear dresses? I told him no, because dresses are not my style, they are useless in functionality, and the dress certainly does not make the woman.
my style is casual, and if I have to dress up I wear a nice pair of pants a sweater or blouse with my dansko clogs(which I love!). You may want to look into getting a Pair of Dansko's they are basicly gender neutral...not to mention comfy.