Before turning the heating rod loose in your safe, pick up an AcuRite digital humidity and temperature monitor for under $10. You want 45-55% humidity. Too little, and, as 1straightshot said, you dry out the wood, too much and you get condensation on the metal and swelling in the wood. I've found that despite living in a coastal, high-humidity region, monitoring the past year and a half, my vault never goes above 52% humidity. That $10 saved me a lot more; I was expecting to need to buy a large dehumidifier.