As for as ich (and Oodinium) is concerned, I always use a systemic approach. Ich is not something that you live with or compromise with.
You eradicate all possibilites of any of your fish ever getting it, period. What if you go on vacation for a couple of weeks?
Infestation of higher order external pathogens (ie not virus, bacteria or fungus and not things that live inside the host) has little to do with stress. It is a matter of chance, related to the unnatural confinement of captivity thus concentration of the patheogen, and the odds are too high to be taken.
I have started to always actively treat for ich for weeks with copper. I have not had a single incidence of the disease since then. Ich can certainly be eradicated but one has to be thorough.
UV only has marginal impact of killing water borne ich. Diatom filter, seldom used now, does quite effectively reduce the concentration of ich to sub-clinical level infestation, but must be used all the time if drug is not used.