Default here.
There is little code anymore that is ui window specific, but at one time we relied on it more. The problem being that any isx scripts that use uicode to work can be rendered useless by a custom ui.
Unfortunately the custom ui setup in eq2 allows providers to needlessly rename windows and buttons, and since this is our reference points to these objects, what I script to work in default wont always work in another UI.
none of the major scripts still rely on it though, so feel free to use what you like. Just understand when you have a bug others don't see, always first try reverting your UI before you complain to the authors.