Keep in mind that there are two different things often called dehumidifiers, and only one of them actually dehumidifies.
The Eva Dry, and other similar devices pull moisture out of the air. The boot dryers and dry rod types of devices do not remove the moisture, but do raise the temperature inside of the safe. Both will prevent rust, but you may not want to use a true dessicant if you are already dealing with humdity levels below 45%.