The sunnah is not to pray the sunnah before and after thuhur, after Maghreb and after Isha when traveling and to pray everything else.

The Prophet (PBUH) never used to miss the two prayers while on journey ie the two rakats before fajr and the night prayer, so if one is on journey he can leave the two rakats after isha and pray the night prayer directly and what if he is not on journey can one leave them and pray the night prayer directly?