I once sold my D30 to a lady whose camera had just died. I sold it with a smaller CF card, and when she filled it, she put one of her own in, and it burned out the camera immediately.
It turns out that this same card had just been installed in the previous camera when it burned out. Fortunately, my camera was just a few months old, and out local dealer took care of sending it to Canon for repair at no cost. She paid dearly to get the first one fixed.
Based on that experience, I would suggest just smashing them good and buying new ones. Its unlikely they will do any damage, but, why risk it.
If you have a old digital body thats only worth $10, you might try them in that, but I'd still say there was risk. If they ever got damp, some dried soap particles might suddenly become conductive liquid and short it out.