Generally it's good not to have all the data one works with checked into version control.
Actually currently no data is checked into version control. When you install BigBang you have to run the collect_mail scripts before getting anything out of the notebooks.
If there's a project that uses BigBang for extensive analysis of data from a single source, then it's probably best to keep that as a fork and have it update from the core repository.
What I'm wondering now is whether all, some, or none of the Summer School notebooks should make it in as is. Currently there are many near-duplicate notebooks in the examples/ directory, along with a lot of other stuff from previous uses of the software.
Some hard work that's going to need to happen soon is pruning and standardizing the stuff in that directory. Along the way we should come up with code quality guidelines and standards for new notebooks.