When a File Maintenance panel is generated, the controls for numeric fields are generated as string fields. There are conversion routines (NUM_TO_STR, STR_TO_NUM) to keep the numeric field and its control string field synchronized. Whenever the record is read, the string field is synchronized, and whenever the user changes the field in the control, the numeric field is synchronized. If the field value is changed by the user entering data or by browsing, etc., the fields are synchronized, so it doesn't really matter which one you use.
However, if you are changing the value programatically, it is important to keep both synchronized, so you should set them both.