Yup, it's not possible to treat the groundwater. If you have a problem with the actual water, you have no choice but to treat it above ground. Most common groundwater contaminants are nitrates, pesticides, and heavy metals. In the city the most common contaminant would be fluoride.
If your well water tests good it should theoretically stay good barring no major changes to your area like new livestock farms or flooding. Talking about the actual groundwater now, not the well piping. That being said the safest way would be to treat all of the water that came out of it. CDC recommends testing at least once per year. Best in our situation to be prepared to test and treat it I suppose, for when things go south.