Do I need to build my story from the top down?

No, you can build either top down or bottom up, as long as you tell from the top down

Stories can be created from the bottom up or from the top down. Stories are, however, told from the top down, beginning with the context, the trigger, the question and the answer before moving step by step to the details.

Very often you will begin forming a grouping storyline at the bottom by sorting ideas into groups, to help identify the themes and messages within the idea set. This process enables you to work toward identifying the overarching answer, or governing idea, at the top of the storyline.

Once this this bottom up clarification (or problem-solving) process is complete, you are ready to tell the story from the top down.