Today user-centered information acquisition over collections of complex XML documents is increasingly in demand. To this end, preferences have become an important paradigm enabling users to express individual interests and delivering personalized information. As the structure of XML documents plays a major part in retrieval, users often have specific preferences about the structure. For evaluation a query has to be unfolded into an entire set of queries filling the structure with more or less preferred values. Since such structure expansions typically contain redundancies, it is important to identify and simplify necessary expansion queries for effective evaluation. To address these issues, we developed a preference query optimizer that not only determines an optimal set of expansion queries, but also preserves the specific ordering induced by the user preferences with respect to Pareto optimality.