I have tuned pianos that have wood soundboards, wood pin blocks, with steel strings, steel pins, and cast-iron frame for a living. Shot steel barrel rifles that had wooden stocks. Own wooden toolboxes that held tools made of steel and brass. In just about in every case if there was a problem, it was high humidity over a period of time. Although recently, I've learned to be careful of certain glues especially the PVAs (white or yellow wood glues) have a gassing off problem.
Many of you have replace those funky plastic handles on your machines with metal. I prefer to replace mine with wood, because it has a wonderful feel to it. The tool that I had for tuning pianos has a rosewood handle. This was not just because it is pretty (which it is, in fact it is beautiful), but because wood is easy on the hand and less tiring than other materials. When you spend two or three hours tuning a piano, it can be quite a strain.
My conclusion is that the real problem is high humidity, and I might add is neglect. Keep these things clean and oiled or waxed or whatever it takes.
