The chlorine in city water takes care of most waterborne pathogens How does chlorine added to drinking water kill bacteria and other harmful organisms? Why doesn't it harm us?: Scientific American I don't know what viruses you'd expect to be in water that wouldn't be in the ambient environment.
I'm assuming if E coli and other bacteria from livestock, vermin, sewage leakage, etc can be transmitted from miles away in city water, associated viruses might accompany them. Since nasal irrigation removes protective mucus from sinus membranes, shouldn't the sterilized water should be as free as possible from any type of virus?