No matter which you choose, please remove it during storage or risk ruining the buttstock finish!
Most damage I have seen on buttstocks from those were from using the rifle while hunting, and being outdoors, and usually amounted to sun damage to part of the stock, while that part under the holder was protected, and didn't get faded and scratched as the rest of the exposed areas did.
I think if the material isn't caustic, storage as a safe queen wouldn't really matter. But then, and safe queen is probably not very likely to sport a shell holder in the first place.
