Would an ablution be valid if only its required acts are performed?
A Muslim needs to perform the fardh and wajib acts of a worship, for which he is responsible, in order to be freed from its responsibility. Sunnahs of that worship conduces to an increase in the rewards of the worship, but they do not cause a responsibility if they are not performed. However, it is makruh not to perform sunnah deliberately (Cf. Ibn Abidin, Radd al-Muhtar, I, 219-221).
Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS