UV steralizers will help control white spot, but will not eliminate it all together. Cleaner shrimp are the same. The only way to prevent white spot 100% is to quarantine all fish before putting them into your show tank. Most people do not recommend treating fish not showing symptoms.
I keep all fish in a cycled QT for two weeks proir to adding them to the tank. If they show no signs of illness, I don't treat them. However, if I see one white spot, I attack with every weapon in the arsenal. Ich is a parasite, and cannot live without fish. Therefore, Ich is not present in your tank proir to being introduced by sick fish. Many fish can carry the parasite but show no outward symptoms. My thoughts on the matter tend towards treating every fish prior to introduction into the tank, however, Dr. Tullock has some very choice words against that. He knows more than I do, so I'll defer to him.