If you're relying on this one feature to detect counterfeits, that's a bad approach. Magnetic ink has been available for years to print onto fake checks (the account digits are MICR -- not just a pretty font)

Side note: One of the techniques that check kiters used to extend the float time was to erase the info encoded into the magnetic ink, which meant the routing & account numbers had to be typed in by hand at the processing center. The introduction of character recognition software means that technique is no longer usable.