Of course it is bad for your eyes! You know the movement which you behave frequently will form the shape of your body especially on your face. You know some people love laugh would have crow's feet. It is the same reason that if you cross your eyes frequently, your eyes will likely to become crossed, the muscle around your eyes will have some change of shape. Your nervous cells will use to control your eyes muscle and eyes ball like this way. Sometimes, cross eyes will make some fun, but if somebody has the cross eyes, that will be a disaster for him. Take care of your eyes, it will reward you a lot!