I believe that with good software designers (and in my opinion all software developers should also be software designers at some level), the magic is in being able to do top-down and bottom-up simultaneously.
What I was "schooled" to do by my mentors is start by very brief top-down to understand the entities involved, then move to bottom-up to figure out the basic elements I want to create, then to back up and see how I can go one level down, knowing what I know about the results of my bottom up, and so forth until "they meet in the middle".
Hope that helps.
