My golden rod type dehumidifier keeps the humidity around 37-39% all year long, which is well below the "rust" level but not low enough to cause the stocks to shrink and crack.
The need for a dehumidifier depends greatly on the conditions of where you live. For example, here in Michigan, the furnace running in the cold months more than negates the need for a dehumidifier. Even if you run a humidifier here in the winter months, chances are the humidity will not get above 50 to 55% unless you get really crazy. Humidity levels below 55% are sufficient to prevent rust from forming on steel.
Since Michigan is the "water wonderland", humidity levels in the warmer months can reach rain forest levels and a dehumidifier is recommended. If you wish, you can kill two birds with one stone by installing lights in your safe. Low wattage tungsten bulbs give off more than enough heat to dry out even the dampest of air, especially in a small confined space that rarely gets opened up to let in fresh air. Even LEDs give off some heat albeit not on the same level as a tungsten bulb. As an aside, air conditioners also remove humidity, so if you live in a warm, humid area, the AC may be all you need to stay within tolerable levels.
If you reside in an exceptionally dry area, you may even wish to run a humidifying system as wood can dry out and crack. Area's where the humidity levels are safe and rarely change negate the need for a dehumidifier or humidifier, while areas where nothing is constant can result in a balancing act.
A golden rod dehumidifier isn't magic, it's just a rudimentary heater that warms the air enough to remove excess humidity. If you live an area that requires the use of such a device, make sure to use a meter monitor the humidity because short of grabbing the device, there is no way to tell visually if it is working, although some have an indicator light, that really only means the light is on, not that the unit itself is actually doing it's job. That's another advantage a light bulb has since it is readily apparent if it is on or not, but once you close the safe door, how will you be able to tell?
I personally don't care for the silica type systems, but that's just me.