Thanks for the comments, guys. My basement is pretty dry and I've never had any problems with mold on any of my leather goods or rust on guns stored there. I have a relative humidity gauge and it indicates around 50% year round. In southern Ohio, we get some severe humidity in the summer, but our air conditioning system is pretty efficient. I've have leather in the safes for years and never experienced any bad effects.
I did have a couple of Randalls with ivory handles. One was a 27 which had a very fine hairline crack in the handle when I bought it. We had some very cold and dry weather last winter and the handle on that knife cracked quite badly. The other knife was a model 26 with no butt cap and it was not affected. However, I was so upset with the cracked #27 that I sold both knives and will probably never own another ivory handled knife.
I've had several handguns with ivory grips and several sets of unfitted grips around for years and never experienced any problem with them.
I am a little concerned about placing the sheaths and knives on the bare shelves. I like the light colored wood because it brightens up the inside of the safe and don't want to cover it with any dark material.
What do you think about applying a good coat of paste wax to the shelves?

- - Buckspen