Speaking purely for myself, I would prefer to have separate mats for the iris and sclera at least, so I can put SSS into the iris but not the sclera. Come to think of it, separate pupil mat as well; I generally set it to be something light absorbant or at least with no specularity at all.

I suppose if the entire eye is one mat you could work with masks to achieve the same thing, but that would make things complicated, so might as well deal with three separate mats.