I think one of the most important things, for me anyway, when building something from the ground up like that is, as quickly as possible, getting the program to a state that you, the programmer, can use it. Even a little bit. Because that tells you where to go next in a really visceral way.
-- Jamie Zawinski, Coders at Work

유저의 입장에서 빠른 피드백을 받는 것이 중요하다. 이게 진정한 의미의 Bottom up approaching 일 것이다.