^ I agree that Grease was set in the 50's, so it isn't 70's style attire at all. Additionally, I'm not sure that poodle skirts and satin jackets would have been considered "fancy" - maybe fancy enough for a high school dance, but certainly not for anything else.
^ Ah, in that case those would certainly be appropriate items to wear to a 50's styled costume party. I misunderstood. I wasn't sure why someone might want to wear a fancy 70's syle dress, but a person never knows!