Right after gathering and prioritizing high-level requirements, get to the User Interface (UI) design. Do this before you complete your MRD or PRD. Yes, before!This suggestion supports one of the reasons for employing an agile approach to product management. You can't specify all the requirements up front; BUFR is inefficient and unrealistic. When you design a user interface, you "test" the initial stab at requirements and uncover additional usability requirements that nobody had considered.
I would go further than Michael and suggest that organizations produce a "releasable" iteration of the product; i.e. work not only through UI design, but also technical architecture, some detailed design, implementation, and testing. (In some cases, GUI mockups can be the output of the first iteration.) See this entry for more details.