Ryan,
absolutely positive. I have a large knife collection that has its own room, humidity and temp. are both controlled. I have been collecting and making knives for over 23 years. This is nothing new to me (not to seem arrogant).
Carbon steel knives will rust much more quickly when they are handled in celluloid. Stainless will also rust, but it takes much longer. If you keep them in a case it "accelerates" the decomposition and rust process. You can slow it down by not keeping them in a box, or a case. However, they will still decompose over time. Also handle "shrinkage" will happen regardless. Some of the newer celluloids may not decompose as fast (due to different solvents being used in the manufacter process), but they are still celluloid basically, hence "no guarantee".
This is just experience talking. I do not invest in celluloid handled knives anymore, not unless someone can produce celluloid that will not decompose. Im still waiting.