great question. and i am going to refrain from reading others replies until after i answer. this is core of crossdressing imo. where do fantasy and reality meet?
i know sometimes for me what is ideal is better until it's real than it loses its power and value.
that said for the most part i'd rather go au natural and be flat or small chested then use any forms... although the few times i have padded it felt like it was taking dressing to a whole new level!