The answer regarding American Walnut is yes and no. I just successfully bent a 20g. Sterlingworth. It was difficult and required more pressure than I am comfortable with and it took two sessions to get it where I needed it to be. It had three inches of drop. On the other hand I have had Parkers and Ithacas that I could not budge. I have bent a few field grade LC Smiths. I guess you have to try and take the risk of breaking a stock. I have been lucky so far.
Cast on cast off is easier and you can probably bend most guns.
How would anyone know if your gun stock was bent? It may have come that way so no, I don't think it hurts value. It opens the gun to a broader market so it may actually enhance the value since both shooters and non-using collectors can both buy it.
