In the last decades a huge amount of knowledge about user interface design has been gathered in the form of guidelines. Quite often, guidelines are compiled according to user interface properties (e.g. usability, accessibility) and/or application domains (e.g. Web, mobile). In many situations designers have to combine several guideline sets in order to address the specific application domain and the desired set of properties corresponding to the application under consideration. Despite the fact that the problems related to the selection of guidelines from different sources are not new, the occurrence and management of conflicting guidelines are poorly documented leaving designers with little help in order to handle conflicts in a rationale and consistent way. In this paper we revise the questions related to selection and management of conflicting guidelines and we propose a systematic approach based on design rationale tools and techniques for exhibiting choices and trade-offs when combining different guidelines sets. This paper illustrates how such as an approach can also be used to deepen the knowledge on the use of user interface guidelines recording decisions across projects in an iterative way.