GRE score submission
A good GRE score can help your chances at admissions; a poor GRE score does not do anything to help your chances, but they can hurt you, particularly if you're already a somewhat "borderline" case.

However, if a school does not require GRE scores, then I would only submit them if they are strong (well above average). Otherwise, you're introducing at best a neutral "fact" into the conversation.