FlushAll is called in rare cases. When the historical data download is completed, when the bar data editor is closed and when the application is closed. It may or may not require specific action for your plugin. As Mark points out, it's not necessary for his database plugin (and I agree), but maybe in a cached situation like a file store when you're specifically managing the file handles, you may appreciate a flush all.
For my MySQL implementation, I didn't find a need for any logic in the FlushAll(). I was able to handle cleanup in the DataAccessor's. However, you're still more than welcome to take a look
if you want a sample.