by Ustadh Abu Yahya.
It was stated explicitly by Ibn Hajar al-Haytami (rahmatu'Allahi alayh) that it is not obligatory. In fact, in al-Fatawa al-Fiqhiyya he mentions that it is not considered obligatory to stick to a single Madhhab, and he even seems to have preferred the view that it is permissible to go outside of the four Madhaheb and to follow the opinion of any Mujtahid Imam, including the Sahaba. He also mentions that this is the favored view of Ibn as-Subki. Ibn Hajar explicitly states: هو حسن بالغ وبه يعلم جواز تقليدهم في مسائل إذ لا يجب التمذهب بمذهب معين خلافا للحنفية
However, this should be restricted to the Talib, who chooses after research, while the general masses should simply ask a reliable Alim for fatwa, which he may follow, whether it is the Mu'tamad or not. In fact, there is a great deal of discussion and differences regarding the rules of seeking fatwa... some super strict (first fatwa received is binding), others extremely lenient (seeking out the easiest option). At the end of the day it is an individual obligation to have Taqwa and it boils down to asking our hearts, in the light of this very same Taqwa, insha'Allah.
'...The layman may follow the Fatwa although it is not Mu'tamad. But in an attempt to stop chaos from erupting, the Ulama have often highly encouraged that the general Fatwa is given upon the Mu'tamad, as these opinions have been checked and verified throughout the centuries, and have been ensured to be solid and grounded in sound knowledge, without the shadow of a doubt. In theory, this makes "fatwa-shopping" harder. And while it has been the preferred opinion by some Usulis, Tattabu' Ar-Rukhas (seeking out dispensations) is NOT permissible in the view of Ibn Hajar.
For the student, it is only natural that one studies a Madhhab, which means it is also only natural that one follows it in ones practice. One only leaves al-Mu'tamad, or the school altogether, after research and conviction, or due to a serious need.
The Shafi'i Madhhab is a Hadithi Madhhab. Consider the following from Shah Waliullah:
قال ابن الصلاح من وجد من الشافعية حديثا يخالف مذهبه نظر إن كملت له آلة الاجتهاد مطلقا أو في ذلك الباب أو المسألة كان له الاستقلال بالعمل وان لم يكمل وشق مخالفة الحديث بعد أن يبحث فلم يجد للمخالفة جوابا شافيا عنه فله العمل به إن كان عمل به إمام مستقل غير الشافعي ويكون هذا عذرا له في ترك مذهب إمامه ههنا وحسنه النووي وقرره
“Ibn as-Salah said: ‘If a Shafi’I finds a Hadeeth that contradicts his Madhhab, it would be seen’. If he has (acquired) the tools for Ijtihad, either fully, or in a particular chapter of Fiqh alone, or in a single issue alone, he has the choice of following whatever he believes to be correct. If he lacks the tools for Ijtihad, and he finds it hard to act contrary to the Hadith after researching it without finding a reasonable answer to the Hadith, he is allowed to act upon the Hadeeth as long as another Imam apart from ash-Shafi’I has acted upon that particular Hadith. This would be an excuse for the layman to leave the Madhhab of his Imam. An-Nawawi and others liked this.”
Wa Allahu a'lam'