The ammonia tag is not that accurate, but it is sensitive, and will show ammonia when it appears 24/7. In an established tank, all you need to know is that ammonia is present, not how much. If you have ammonia in an established tank, you have issues that can snowball quickly as time advances, so monitring his tag is a smart thing to do to prevent such scenarios.
as far as low level nitrATES go, I like the Sal;ifert kit the best of those available for hobbyist use. If you see ammonia in an established tanks, big water changes, ammocarb, amquell and nitrates tests are indicated, as well as cranking up the skimmer and your
calcium reactor flow rate if so equipped.