My homeopath I took a class from didn't recommend them, but said that if you were going to do them, you would most likely benefit from only taking them in the event of a breakout in your area. So, if you where in a situation where you might be exposed to something, then your homeopath would find out the best remedy for most of the people who were being effected. Since not all people have the same symptoms when they get sick, not everyone would be protected by the same remedy, but the majority would.
Taking a nosode years before you might come into contact with something really would do you no good. It would really have to happen just before for it to work.