It's commonly thought that epistemic normativity lacks the structure of moral normativity. We should believe in proportion to the evidence, and that's that. But most moral theorists will carve things up further, e.g. into the impermissible, the (minimally) permissible, and the supererogatory.
I'm not convinced there's any great difference here, however. In both cases, we ought to believe/do whatever we have most reason to believe/do. Nevertheless, we can distinguish between reasons that are more or less stringent, and failures or imperfections that are more or less forgivable. This is so even in the epistemic sphere. Some biases or errors in reasoning are worse - more blameworthy - than others. Young-earth creationists, holocaust deniers, etc., might be considered especially egregious offenders, for example. Moral and epistemic offenders alike may warrant censure -- blame in the one case, and ridicule or scorn in the other, perhaps.
Further, it seems pretty clear, in both ethics and epistemology, that not all imperfections (or failures to believe/do as we ought) call for such censure. There is a significant range of 'permissible', or tolerable, belief and action -- even though within this range, some options may be determinately superior to others.
I think we can even extend the notion of supererogation, or going "above and beyond the call of duty", to the epistemic sphere. Consider such ordinary optimistic biases as that of parents exaggerating the virtues of their children. What should Mother believe about the aptitude of her little Joe-Average? She certainly shouldn't consider him a genius -- that would be a bit too unhinged. But she may be forgiven for considering him a little above average, even if he's not (and the evidence is sufficiently clear about this). Indeed, we probably can't reasonably expect or demand much more from her than that.
But now suppose Mother is unusually dedicated to overcoming her biases, and reasons thusly: "Joe seems above-average to me, but mothers are known to overestimate such things, so I really should compensate for my own bias here. This suggests that Joe is really about average." Here it seems to me that Mother is going above and beyond the call of (epistemic) duty. A normal, adequately reasonable agent would probably have just stuck with their conveniently over-optimistic initial judgment. But she really went the extra mile. Doesn't that sound like supererogation?