Should I wear a strapless dress to a Bat Mitzvah?

I am attending a Bat Mitzvah in a few weeks. I'm going to buy a dress to wear to it and was looking online. I found one that I really like at a store in my mall. It is strapless and comes about 2 inches above my knees. It is my best friend's Bat Mitzvah and I don't want to insult her by wearing an improper dress. So, should I wear a strapless dress or a shrug over it?