I find this rather odd - the sun is essentially a point source, and the reflection of the sun in an eye would certainly be a point!
The most common large-but-bounded diffuse light source is rectangular: A window.
Illustrators and cartoonists often put in a reflection of a window to show "roundness", so why is it wrong to do it in a photograph?
I asked a local portrait photographer (a good one, won lost of international prices and stuff) about this, and he couldn't answer...