Everyone has there own opinions, but for the most part (besides of course what has been verified, etc.) the rest is all theory.
I think safety deposits are fine, as long as its not located in an erratic climate area, either too cold or especially too hot moist. But 95% of safety deposits in the U.S. are "okay."
I think the absolute biggest thing is to just make sure your books are regularly removed for a short time, for example when you take them out to enjoy for a day or so anyway, about 2 times a year.
The biggest problem with keeping anything in a tight enclosed space for too long, is that IF there is something in the air (a small chemical, etc.) it will continue to compound overtime. So if you left something in a safety deposit box for a decade and had not removed it once, I would not be surprised if something could have happened.
This is all my "amateur" opinion however, and am open to correction.