The default design view representation for a document should be an option that can be applied to a single design view representation in the document (perhaps right click and check as default).
The default would not change when a different design view representation is activated (possibly a system setting to retain existing behaviour). It would be used for the preview picture when the document is saved and would be the active design view representation when the document is reopened.
At present, our users need to remember to switch back to a default design view representation when they do a final save. With regards to when a document becomes dirty and requires saving, it segregates the activation of a design view representation from selecting a default.
Regards,
cadull