It really depends on what you're keeping.
If you have a fish only system then you really only need to test for Nitrate, PH, and Salinity. A cycled system should never have detectable ammonia or nitrite without a death occurring to cause a spike. if you're still getting ammonia readings after the tank has completed it's cycle then you're probably feeding too much.
If you're keeping corals, specificly SPS, LPS, or Clams then you need to test for Calcium, Alkalinity, and Magnesium as well.