Read the instructions related to whitener carefully. If there is no instruction about whitener on OMR sheet, then you can use whitener. The Addmen OMR software result can be accurate, even after using a whitener on the sheet.
But sometimes very thickly applied whitener gets chipped off while stacking, storing or bending the sheets during scanning. These white chips may fall on the scanner lens and may create a virtual shadow mark on your sheet image. This fallen chip may also create an impression on many of the sheets following yours sheet in the scanner. To avoid this trouble examination bodies instruct to avoid the use of whitener.
If the whitener does not dry properly, then it will stick to other sheets and two different sheets will adhere to each other. These sheets may get damaged while separating. Then same thing can happen to duplex sheet and it will create unwanted marks on another sheet. Thus to keep everything simple and safe, use of whitener is prohibited.
As we mentioned, if an answer has been properly whitened, technically it won’t appear as a black mark in the scanner and thus there is no issue in reading such sheets from the software. So your purpose would be solved. But again the use of whitener would give birth to a situation of contention that who actually marked the whitener, the candidate or the examiner. To avoid this botheration of legality the examination bodies instruct not to use whitener.