Is it obligatory for the congregant to follow the imam in matters of disputed rulings that do not affect the validity of the prayer, or is there no harm in not following?

It is better for the ma'mum (follower) to perform the sunnah acts that the Imam omits, as long as it does not disrupt the following, such as qabd (placing the right hand over the left) and raising the hands during the takbirat (pronouncement of Allahu Akbar) of transition. As for the sunnah acts whose performance would result in disrupting the following, such as the prostration of recitation (sujud at-tilawah) and the first tashahhud, the ma'mum must follow his Imam in omitting them.