Wa `alaykkumus salaam wa rahmatullaah
There is difference of opinion in this matter
According to the Hanaabilah (The Madhab of Imaam Ahmed rahimahullaah) is that it waajib to say Rabbigh firli atleast once and the rest of the Takbeerat, sami `Allaahu liman hamidah etc (except for the first opening Takbeer), however there is close to an Ijmaa` that it is not waajib, but rather mustahabb as it has been authentically reported on a number of sahaabah that they left out all the other takbeeraat (except for the first one), this is the view of the other 3 imaams (Maalik, Shaafi`ee, Abu Hanefaah) and the latter seems to be the correct opinion. However one should strive to always say it is one of those sunnah acts that the Prophet :saw2: never left out.
Wa salaamu `alaykkum
P.S What I mean by Takbeerat is not raising the hand (this is sunnah) rather saying Allaahu 'akbar