You can get relatively precise measurements using LaMotte or Hach test kits, although they are considerably more expensive (~$50+ per kit) than average kits. But the run-of-the-mill test kits that you normally see at your lfs
can only give you ball park estimates of your water conditions. They have an especially hard time giving accurate results at low concentrations. I would recommend using nitrate and phosphate kits to track nutrients (Seachem makes pretty decent low end kits), and also pH and KH
kits to give you an idea of CO2
and alkalinity levels. And like jsoong said, avoid the stripe kits completely.