I have never de-magnetized my MC cartridges.
Never felt the need, as I never heard any "veiling" occurring from magnetizing effects.
Also, Van den Hul specifically states this is not a "safe" thing to do to your cartridge, as repeated de-magnetizing can weaken the power of the magnets, and actually cause the loss of resolution that you are trying to prevent by using a de-magnetizer. He also may reject your cartridge from his cartridge rebuilding service, if you have repeatedly done this to your cartridge.
While I am not sure that this is a big problem, I would not consider this a prudent thing to do, unless I had a noticeable "veiling" effect, and it was worth a try to reduce it. In any case, I wouldn't do it, but maybe once during the life of the cartridge, if that.
Others may disagree with me on this, but I don't hear this problem happening on my systems, even though I know it supposedly "sneaks up on you".