That's one of those "Whatever works" things and will mostly depend on the workflow how the actual app is created. Depending on how that goes, you may need to structure your doc specifically to export your customized items as PNGs or SVGs to be used in the programming environment, while standard UI widget would not at all be relevant and could be some big blob somewhere. It's really not a matter of an academically "correct" workflow, more a case of making things easy down the line...
Mylenium