I've been the past few years. You really don't have to worry too much about the dress, because there's always some people who are just in regular cosplay, some people in massive formal stuff, some people in semi-formal stuff. Just don't show up in like, jeans and a tank top (although I've seen someone where jeans to it before XD I don't think they were supposed to)
I had fun at the dance both years in a row. I'm gonna be Roxas and wearing a black and white dress with my Organization XIII coat over it, but not zipped.