Iomaca's exactly right, it depends what you want to do with the database.
I'm a database programmer so I have a dedicated database programming setup to work with and I keep everything (datawise) I can get my hands on - even if I don't need it right now.
The simple answer I would think is keep whatever data you would use if you were working a manual system and you'll soon find out what else it would be nice to have. Then you can expand.

KV