A well-known rule of thumb claims, it is better to scan than to use an index when more than 10% of the data are accessed. This rule was formulated for relational databases. But is it still valid for XML queries? In this paper we develop similar rules of thumb for XML queries by experimentally comparing different execution strategies, e.g. using navigation or indices. These rules can be used immediately for heuristic optimization of XML queries, and in the long run, they may serve as a foundation for cost-based query optimization in XQuery.