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Any time a field referenced in the calculation is modified. Stored calculation fields display values more quickly and make for faster sorts and finds. On the other hand they slow down the import process when you import records as the stored calculation fields will need to be evaluated record by record as each new record is imported.
Since my databases will perform sorts and finds many times more frequently than data imports and I can perform imports via script during down times when users are not accessing the database, I prefer stored calculations over unstored whereever possible.