Given a retina display aims to copy the sharpness we see in our every day lives from real objects, I would suggest it doesn't intrinsically carry a risk of tiring your eyes, in the same way as we don't get tired seeing all day long even though the edges of everything we look at are generally sharp. Rather in the sense of drawing on a drawing board, or reading books, ones eyes do get tired. Given that OSes are in their infancy when it comes to giving us the tools to properly manage size and resolution, it could be that some tasks are more tiring than they used to be.