Is the opening supplication prescribed in supererogatory prayers such as Shaf' and Witr and the emphasized Sunnah prayers, and what is the ruling on one who omits it by mistake or intentionally?

The majority of scholars hold that the opening supplication (Istiftah) is sunnah (recommended) in prayer, whether obligatory or supererogatory. Nothing is required of one who omits it, whether intentionally or forgetfully. However, one should not intentionally omit it, due to the great reward that would be missed.