Sorry, not sure I understood the question fully - but I'll answer what I think you asked...
When you do a water change, you have to decide for yourself how much water to remove from the tank. It all depends on the volume of water and how many fish you have in your tank as to how often and how much water you need to change. Obviously, if it is looking quite dirty, smells bad, or if fish are showing any signs of stress or illness (from toxin build up) then you remove more water.
If your tank is kept stocked with fish at a decent level and you keep to a regular maintenance regime, then your tanks will not become overly dirty/smelly/toxic. In fact, once you are into a good routine, your tank should never reach the stage of looking rank! Regular good maintenance should mean you have a clean, clear tank with healthy fish.
When you remove, say, 15% of tank water, and replace it with fresh dechlorinated water, it's not necessary to test the water in the tank every time you do a water change because you are not putting anything toxic into the tank that would raise the chemical levels. If anything you are then diluting whatever trace of chemicals are in the tank already.
Of course, if you wanted to test the tank water after every water change that's entirely up to you. I suppose there are odd occasions when water companies put things in the tap water that can have a detrimintal affect on the fish. Most of us wouldn't realise until our fish showed signs of stress. If you wanted to test after each water change I guess you'd know in advance if anything was wrong.
There are no set rules really, you have to find a routine that you find suits you (and your wallet!). So long as you are doing the main things to keep your fish happy and healthy, then that's all that matters. If you have the time and money to test every day or every week, indefinitely, it's doing no harm. There are some who do this and keep daily records of readings.
Personally, that would take the pleasure out of fishkeeping for me. I test when I need to test now, not just for the sake of it. Besides, with a recession on I couldn't afford to keep buying test kits every few weeks LOL
Athena (sorry, I'm really rambling a lot today)