Well, my friends have asked this question from the doctor, and he has told that they aren't really bad for you. And if you just gaze at them for only a few moments and either see the hidden image or give up, that can be fine for your eyes. But on the other hand, if you stare at them for too long, and your eyes will become strained as they concentrate too hard on seeing the hidden image. In that way, it will just increase the eye strain, causing the tired eyes. And then it will just be bad for your eyes, weakening your eyes.
Hello, in my opinion, stereograms are actually good for your eyes. They can help you rest your eyes and relax your eyes. There are a lot of positive physiological affects for the eyes from viewing stereograms. But you can not see them for a long time. If you got tired eyes from seeing them, that will be bad for your eyes. That may help.