It was at first, I needed to wear more feminine clothes to feel better with myself. No dresses cause they were looking odd on me at start.
Now that I can pass even naked (except for not having GRS yet), it doest not matter anymore. I'm more the type to wear a tanktop and jeans.
Everything that could make me look more like a woman was the welcome before. Now if I wear a man t-shirt (still use my old ones when going to sweat like hell), peoples tell me "ooh, you stole your boyfriend t-shirt?" so I care less about clothes I wear, I just try to be comfortable and like myself in the mirror.
I had to wear dresses in the last two months and I now look sexy in it (well that is what I've been told), but I still won't wear one on a casual day it's not my style.
Short answer :
It was damn important at start, it's not anymore.