I wouldn't go so far as to say that automatic writing can he dangerous, however if practiced improperly you can experience a recoil effect where you go from being in a trance like state of mind to being fully conscious and aware of your current whereabouts. In order to prevent this, one needs to work on their transition skills so that you know how to enter and exit the trance/altered state of consciousness with ease.
When channeling anything, be it in the form of friend or foe, it is wise to take the necessary precautions so that you won't be influenced when you've finished your auto writing. There's more than one way to accomplish, and using crystals that one is familiar with is on that list, however I wouldn't recommend solely depending on your crystal. Go through the fundamental stretches, and by that I mean stretch your mind muscles not your body muscles. Before you begin remember to maintain your shields, balance your grounding, and go into it with the right state of mind.