At 1/72 scale, I paint more of a line for the eyes, or I'll even cheat and use a wash to pick out sculpted detail. But I don't worry about trying to paint the eyeball itself. Others have mentioned scale and distance, and 1/72 scale figure I've painted have been for dioramas. The viewer is actually a hundred scale feet away or more, and at that distance, you don't really see the whites of someone's eyes, unless the person is surprised. As Shep wrote in his "Tips", people outdoors tend to squint.
Same goes for 1/48 scale, my favorite scale for airplane models, when I paint the crew figures. I don't worry about painting the eyeball.
At 54mm, I'll add more detail, because the figure is larger, so the viewer is "closer". At that size, it's worth it to add that detail, and it's not too difficult. Even if I just lay down a light gray for the eyeball, once I have the upper and lower lids painted, that little bit shows.
I also build 1/20 scale figures. At that scale, I find it necessary to include that bit of detail.
Hope that helps, prosit!
Brad