Which camera ? On some cameras this is already correct, depending on camera shooting mode and which override you are interested in. (On some cases, it can record the override value even though it wasn't actually used by the camera...)
Usually the canon "maker note" is more likely to be accurate than the regular exif.
CHDK doesn't write directly to the exif. Whether the camera records the override or not depends on how the override was hacked into the the shooting sequence, and this varies between cameras.
