Should a 3rd party app register itself with the admin site?
I've written a custom application which, like django-tagging, can be integrated well with other applications in a project. Is it considered a good practice to register my models with the Admin site by default? Could this possibly break anything?
It depends on whether or not the majority of the subset of your potential users who use the admin will want to use the admin to interact with your app. If they will, register it. If they won't, don't.
I'd probably err on the side of registering personally.
Users can always unregister them if they want: