It cuts down the chance significantly. Ich can of course be carried into an aquarium via Live Rock or Coral but it is unlikely. LR gathered from the ocean and then sent has a ridiculously low chance of carrying Crypto tomonts. Coral that has been house with other unmedicated fish in captivity would have a higher chance but it is still low considering the tomont can only attach to the coral base plug.
If you eradicate the parasite from your system then it can't magically appear.
If you eradicate the parasite from your system then it can't magically appear.