Does a daughter need to listen her father on a subject: he order her not to cover her face? What reasons can influence a person to leave his Sunnah? Our Imam said that it is permissible to leave some Sunnah in the aim "not to frighten weak or new Muslims". Is this correct?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

According to the preponderant opinion of the Muslim scholars, covering the face is compulsory for a Muslim woman to protect herself from temptation and from men as well. So, it is not permissible for any Muslim woman to uncover her face to non-Mahram men or to listen to her father in this regard since none deserves obedience when ordering the disobedience of the Lord. The obedience of the creatures is restricted to lawful matters only as the Prophet said: “There is no obedience in disobeying Allah; verily obedience is only in what is permissible (and reasonable).” [Al-Bukhari and Muslim]

However, leaving some desirable acts for any greater benefit is permissible; otherwise, leaving them is counted as negligence in good deeds. Shaykh Ibn Taymiyah said: “Performing a deed sometimes becomes desirable while at other times neglecting the same act becomes desirable based on the advantages that result. A desirable act is sometimes neglected based on the pieces of evidence of Sharee’ah to avoid a greater harm as the Prophet did not rebuild the Ka’bah on the original foundation that was established by the Prophet Ibraheem .” Ibn Taymiyyah continued saying: “For this reason, Imam Ahmad and others recommended that an Imam in prayers should ignore what he believes preferable if the followers believe otherwise, only to unite the hearts of Muslims and keep love and affection among the Muslim community. For example, if an Imam is of the opinion that the prayer of Witr should be performed separately, (i.e. performing even Rak'ah separately and then performing one odd Rak'ah) but he leads a group of Muslims who believe that the Witr prayer should be performed continuously with one Tashahhud, (i.e. without separating with Tasleem) and the benefit of doing so is greater than separating it, then it is better to perform it without separating it. The other example for neglecting what is preferable is the way of reciting Bismillah at the beginning of the prayer. If an Imam believes that it should be recited loudly while the followers believe the opposite (i.e. that it should be recited silently) or vice versa, then it is better for him to do what is more effective to keep the hearts of Muslims united.”

Allah knows best.