It is Mustahab to wipe the nape.

Sayyidna Abdullah ibn Umar, Radi-Allahu anhu, narrates that the holy Prophet (saw) said, 'Whoever performs Wudhu and passes his hand (Masah) over his neck (nape), will be saved from (the punishment of) wearing the yoke on the day of Qiyaamat.
(Daylami; I'elaaus sunan vol.1 pg.121; Idaaratul Qur'an)

Allamah Ibn Hajar Asqalaani (Rahimahullah), a famous commentator of Sahih al-Bukhari writes in his book Talkheesul Habeer (Vol. 1: p.92) that this narration is Sahih. Allamah Shawkani (Rahimahullah) has also confirmed this in Naylul Awtaar (Vol. 1, p.204).