To me, if you're not hurting anyone, go for it. That's always been my belief. I'm a non-conformist with a lot of things, especially the way that I dress. I treat others with respect and they do the same for me.
I have piercings, dyed hair, tattoos, and I wear slacks to church instead of skirts. I'm Mormon. No matter how I look, my people are good to me. Outside of church, I wear tank tops with my jeans. I don't consider it a bad thing at all. Just me being myself. I understand how some people view my style, but if I'm not dressed like a total floosy, I don't need the fuss.
Use your best judgment on dress. Just from my perspective, if it were my daughter, and she wanted to wear a prom dress that wasn't exactly church friendly (IE: strapless or thin straps) then I would definitely let her. It's prom, if the dress was still elegant and not trashy, then I would be fine with it.