Thought I'd chime in with some additional info. When setting up a new tank, or moving to a new place, etc., it's important to establish a "baseline" of how your water looks from one week to the next & from one water change to the next. I will assume this is after you've cycled your tank. After that, testing is pointless, and is a waste of time, effort, and money.
The foundation of good fishkeeping is keeping things the same and routine (after optimizing conditions). If something changes, such as a water supply, nutrient supply (for planted tanks), feeding habits, etc., then such change should warrant a testing. I never test regularly once I've "set" something. However, I do randomly test once in a while just to make sure nothing strange occurs. It never does. I guess I want to use my test solutions up before they expire...
Like many have mentioned, watching the fish is a good sign. Most fish are very vigorous and active under optimum conditions. When something is amiss, they become a bit sluggish.