Don't do this too much.

Your eyes have to focus. For things close enough they can tell from the required small angle of cross-eyed movement, how to focus.

These pictures (and stereograms) disconnect the focus and angle of your eyes. Do it enough and you loose the ability to focus quickly. I found it out playing with steregrams on a 286 pc a long long time ago.

YMMV but I lost the ability to quickly re-focus object that you dont already know the size of; and it's a bitch.