I modify several fields to use the data types I want. Later I modify the query and add some more fields. If I let iReport get the fields automatically then it reverts all of my changed data types. So I either have to re-modify all of my data types, or I must manually add the new fields.
It would be an interesting enhancement to make it easier to keep my changes. For example, iReport could store (I don't know exactly where it should store this) the original data type if I manually change it. Then when I click "Read Fields" in the future it could set the data type only for new fields and for fields that had not been modified manually.