It is quite strange though, since I rarely see someone who try to cross his eyes for fun to be honest. As you know , our eyes are the windows of our mind which really deserve your due protection, not abuse. Yes of course, by doing so, there is high risk that your eyes might be infected because they are allergic to bacteria,which is resulted from dirty hands.
Yes, it is not bad for you to cross them. When you want to look something clear, you must get your eyes crossed. Though it will be easily for you to watch more carefully when you do this, it will occur double visions as well. If you just make eyes crossed for fun, it will not do great harm to your eyes. But remember don't do that often, because it may cause eye stain and give pressure to your eyeballs. It may lead to vision problems as well.
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!