I think the more appropriate question is: *given your research interest*, where should mixing of methods occur? In principal, you can have many different forms of exploratory mixed-methods designs and where mixing occurs depends on what you want to find out and in what way. If you are interested in the opportunities of mixed-method designs, you might check
Leech, Nancy and Anthony Onwuegbuzie (2009): A Typology of Mixed Methods Research Designs. Quality & Quantity 43 (2): 265-275.
or one of the textbooks on mixed methods by Creswell.
This is an interesting question. My practice has been that incorporate Mixed-Methods throughout the research design. Form a pedagogical standpoint, I encourage my community of learners to wrestle with implications of using approaches. Since I teach in a small department that encourage transdisciplinarity, it is unlikely that those in my classroom have a robust background in soc.scientific methodology/research design-often coming from history, religious studies, ethics, biblical studies. I have found so far, that incorporating mixed-methods at every stage in the learner’s progress has been helpful in facilitating a more comprehensive understanding of methods. My to go texts have been Hesse-Biber’s “Mixed-Methods,” and Vogt, et.al “When to Use What Research Design.”