While most bites may not result in injury, there are reports of bleeding after bites. In rare occasions, when the fish is large, the bite can result in serious damage to the fingers. Arowana have powerful jaws which are designed to catch amphibians and even birds.
Hence, keep a safe distance from the mouth. If you have to remove or adjust anything in the tank, it is better to use tongs or similar tools. Move the tool gently so as to not startle the fish.