I imagine that depends on who the design document is for, and who pays the bills, and whether or not they care about OO vs function-oriented programming. And, of course, what's best for the app itself....
But you need to get some input from somebody besides me. I am a hacker with no formal training whatsoever, aside from writing little annoying programs in Apple's PRODOS years and years and eons ago in junior high school, heh......