In my experience Top Down is where you start with Reports that need to be filled. These in turn dictate the design of your cube, data mart and underlying relational schema. Bottom up is starting with a list of capabilities that are needed which you then can build your Relational schema off of then cube and reports.
In my experience top down is faster initially because you can quickly meet the needs of the orginization, but typically forces a design that may not be as flexible making expanding the capabilities of the solution more difficult.
It is an easy trap to get caught up in. My experience leads me to believe you get forced in a top down method when people do not understand the technology or methodology and are unwilling or unable to learn.
I find myself quickly going from a bottom up to Top down environment yet again.