@algoatall it entirely depends on the degree of realism the artist is going for.
if it's fairly cartoony it's fine and can even be preferred to help get across an expression, or sometimes being able to see both eyes can be cuter because big poni eyes.
If it's a more realistic style then you probably shouldn't see it.
I vary from piece to piece based on how far I am taking the rendering.
How much of the eye is covered effects things too, if it's barely covered it's usually fine to show the eye.