Style - When to serialize a Django model Instance: signals vs model's save method
Post save. That way the new data (the reason for performing the serialization) is already in the database. It'll make for a much cleaner bit of code that simply takes from the database and doesn't have to worry about adding an extra value.
The other way that comes to mind is to maintain the xml file in parallel to the database. That is to say, in your save() add the data to the database, and to the xml file. This would have a much less overhead if you're dealing with huge tables.
I plan to serialize a Django model to XML when it's saved or updated.  (The XML's going to be imported into a flash movie).  Is it better to listen for a    post_save() or       pre_save() signal and then perform the serialization, or to just handle it in the model's    save() methon