I have been to many comps and I have never seen anyone wear a strapless dress. If the dress doesn't stay put, you can be pulled off the floor and told to change. I have a friend who was wearing a dress with one over -the -shoulder strap. The other side slipped down a little too low and the scrutineer told her to change. Look at ballroom dresses on ebay. There are a lot of choices and some very good prices. There is also the possibility of buying a used dress. As Clary said, you don't want to wear anything that screams 'newbie'.