According to modern Usuli school it is not permissible to do taqleed (of the mujtahid) in Usul-e-Deen. However this was not the view of all the Usuli scholars in the past.
Here we will look at the thoughts of one of the oldest and most renowned Usuli scholar Shaykh Tusi (of 5th/11th century), compiler of two of the four canonical collections of hadith (kutub-e-araba). In his famous book “Uddat al Usul” he argues that taqleed should be promoted not only in furu (branches) but should be extended to include usul (beliefs) as well. Note that usul includes not just tawhid, risaalat and qayaamat but also includes belief in angels, satan, raja’at, wilayat, creation of the Imams (from the clays), the hierarchy of spirits, nature of ilm, the night of power, taslim, taqiyyah, faith and works, status of the sinner, intercession, kalam, badaa (predestination), free will, vision/nature of God, etc.
Tusi argues that the ignorance in usul of the layperson not guided by a mujtahid is more likely to expose him to error than the possibility of following an errant belief (usul) held by a mujtahid. In other words, the layperson is better off following the beliefs (usul) of a mujtahid even if some of those beliefs turn out to be incorrect on the day of judgement, because not refering to the mujtahid for beliefs at all is even more dangerous. He therefore extends the doctrine of taqlid to the usul and as a result to the legal probabilism (meaning the usuli ijtihad based on zann [conjecture]) that goes along with it. Below is an excerpt from his famous book “Uddat al-Usul”:
The opinion that seems strongest to me is that the muqallid has the right (haqq) in the ‘roots’ (usul) of religious matters. Even if he falls into error through his taqlid, he shall not be punished for it, but pardoned [in the same way that he is pardoned for errors having to do with the ‘branches’ (furu)]
For I find not one person among the Shi “a who gave his allegiance [walaa] to any of the Imams toward whom the Imam then terminated his responsibility [mawala] because he had [merely] heard their words and then adopted their beliefs, not relying on any proof based on reason or revelation.
Sheikh Tusi argues above that when the Imams (as) were physically accessible people did taqleed of them even in Usul i.e. accepted their words (on matters of beliefs) without asking for proof. Therefore, the layperson should do the same for the mujtahids as it is a safer option. He continues:
Nor can anyone say that [taqlid in usul] is not permissible because it leads to enticement [of the laity] to [beliefs] that might be foolish — for it does not lead to anything of the kind. It is impossible for a muqallid to conceive, unaided, that such a thing [i.e. knowledge of the usul] is feasible for him.
Rather he fears [attempting] to approach such knowledge! Neither is it possible for him to be aware that the punishment of God would fall away from him [if he were to hold the correct beliefs] and sustain belief—for he could only be aware of this if he had knowledge of the usul [in the first place].
But if we assume [as I do] that he is a muqallid in all these things, then how could he be aware of the dropping away of the punishment of God and be [at the same time] enticed to belief that might be foolish, or how could he sustain [such a false belief]? Someone other than himself—from among those learned men who possess knowledge of the usul and who have studied all the matters relevant to it—would know [of the falling away of the punishment of God from those who hold the correct beliefs], and learned men do not terminate the responsibility they have toward their charges, nor do they abandon them.
It is not possible for them to do such a thing unless they know that the possibility of divine punishment has dropped away [from those for whom they are responsible].
This puts the muqallid beyond the reach of enticement.
[Source: Uddat al-usul – Shaykh Tusi, 115]
In the above excerpt, Tusi points out a circularity in the argument of those who are against taqleed in the usul — awareness of the danger of divine retribution which supposedly leads the “ignorant” (jahil) layperson to seek knowledge of the fundamental beliefs itself depends on knowledge of those beliefs.