We always (ALWAYS) treat our water. It is city tap water that is some of the finest, purest water in the world. However, that doesn't mean that it is the best brewing water. It is very low in all minerals, so we always need to add calcium (Ward labs says it is 5 ppm out of the tap). Depending on the brew we may also need to add sulfate (Ward labs says the water is 1 ppm out of the tap) or chloride (Ward labs says 4 ppm out of the tap) and usually some acid if the grain bill is not very dark.
I am very happy that we have such good water, but I know that there are others who are not so fortunate. I understand the need to go RO and add stuff back in. I add water additions as the water is heating (usually around 120 - 130 F) because I know that some things don't dissolve well at lower temperatures. If the water becomes a bit cloudy at that point I don't worry. Cloudy water doesn't necessarily result in cloudy beer.