When using the JS library for a mobile app, do I still need utag_data defined in the app, or populated for every screen, or pass all data needed in a utag.view call for every screen view? I don't see many examples on-line so any best practices or examples would be helpful.
I assume you are running more of a mobile web app and not a native app? There are SDK's available for native apps and a pretty good walkthrough of how that process works for both iOS and Andriod here:
But from what it sounds like in your question, you are looking more for a mobile web approach, which is pretty much the same approach you would take for a normal web implementation with a few configuration exceptions that you can examine in more detail on the Creating a Mobile Profile page:
Hopefully that helps!