There are many Sunnahs which are regarded as Maukida, whereby it has been regarded as important in Shariah and to miss it even once without reason is worth reprimanding, and to make a habit of missing them would mean being a wrongdoer (Fasiq) who’s giving oath would not be counted and is worthy of hell.
C141.1 The Sunnah-e-Maukida are these;
a) The first two Rakats of Fajr, before the Fardh Salaah
b) The first four Rakats of Dhuhr before the Fardh Salaah and the two Rakats of Dhuhr after the Fardh Salaah.
c) The two Rakats after the Fardh Rakats of Maghrib
d) The two Rakats of ‘Isha after the Fardh Rakats.
e) The first four Rakats before Fardh of Ju’ma and the four Rakats after Fardh and it is better also to pray the two Rakats additionally after Fardh [Guniya, Hidaya].
The most important Maukidah is the Sunnahs of Fajr, upto the extent that many Ulema regard them as Wajib. Therefore they cannot be prayed without reason whilst sitting or on a vehicle or moving car [Fatahul Qadir etc.].Rules of missing Sunnahs
It is not allowed to start any Nafl or Sunnah once Jamaat has started. The only exception is the Sunnahs of Fajr and the rule is you must start them even if the Jamaat has started as long as you are aware that you can finish the Sunnahs and join the Jamaat even if it is in the last Qaidah. You must pray these Sunnahs separately away from the rows of Jamaat as to pray them in the same row as the Jamaat is not allowed.
Four Rakaat Salaah before ‘Isha and ‘Asr and four Rakats after ‘Isha all with one Salaam is Mustahhab. It is also the case if you pray two Rakats after ‘Isha then the Mustahhab will be fulfilled. It is also Mustahhab to pray four Rakats after Dhuhr. It is quoted in the Hadith that those who pray these will have the fire of hell made Haram for them.