Answered by Shaykh Ali Hani
Is the tawarruk only recommended for the obligatory prayers except salat al-fajr or is it recommended for every last sitting (including the nawafil (supererogatory prayers) of two rak`ahs) before giving final salams?

It is sunna to sit in the tawarruk style after the last rak`ah of all prayers (obligatory or supererogatory). Imam Nawawi states in al-Majmu`, “It is in the texts that it is recommended to sit in the first tashahhud in the iftirash style, and in the second [s. tashahhud], the tawarruk style. If it is a prayer with two rak`ahs, one ends with the tawarruk style.
And Allah alone gives success.
Ali Hani
Kharabsheh, Amman, Jordan.
[Translated by Sr. Shazia Ahmad]
Note: The iftirash style of sitting, as well as the tawarruk, are explained and illustrated in the Reliance of the Traveler (pp. 139, 141).