Questions like these are always out-of-context because it's assumed that the nature of a fish or plants response is dependent on the absolute value of that parameter, when in fact, it's usually dependent on the phenomenon in the water that is causing the parameter value to change.
As Darrel mentions, dirt and pollution in the tank will always cause a rise in TDS. Pollution is bad for fish and plants, so if there is a TDS instability due to increased pollution, then of course this is bad. If the TDS increase or instability is due to the addition of nutrients, for example, then this is not a big deal because nutrient addition does no harm.
Adding food to the tank causes a TDS rise. Is this good or bad? Food is required by the fish, so there must be some good, but the pollution and Oxygen loss that results is bad. So is the good more important than the bad? If so, then by how much? If we focus on the good/bad relationship of all components we add to the tank we will see that everything is a compromise, because all components have some degree of good and bad. So the answer to the question for this component is that we must manage the amount of food input, and concurrently remove the pollution caused by the food addition. So in this sense it may not really matter that much what the actual TDS number is, but that the component of the TDS that is due to food pollution is minimized, is a much more important approach to fish keeping than blindly following some arbitrary number, because fish are killed by pollution, not by some arbitrary TDS number.
It's exactly the same story with pH. The pH Cultists will swear by some bogus chart showing what minimum and maximum pH values are necessary for some particular fish, yet there are plenty of successful hobbyists out there who routinely break those rules, while concurrently, the Cultists continue to experience failure while they vainly attempt to keep pH constant. If you add CO2 to a tank the pH will fall. When the gas is turned off at night the pH will rise. CO2 is toxic, so if you add too much you will annihilate the fish, but the lethality will be cause by the CO2 ingestion, not by the pH being too low. Many people foolishly attempt to maintain the mythical pH stability by actually adding toxic chemicals to the tank such as "pH Down". These acid buffer products are extremely toxic. Normally they are composed of some strong acids such as phosphoric or Nitric Acid. So the hobbyist dumps loads of this stuff into the tank, which only manages to drop the pH for a short time, because the acids combine with other compounds in the tank over time and are somewhat neutralized. But the toxicity is not neutralized. The pH rises, and the hobbyists responds by adding more toxic acid. Sometimes the hobbyist perceives that the opposite "problem" is in the tank. The pH seems too low, so they add "pH Up" which often is just a bicarbonate product, and is sometimes Sodium bicarbonate. Sodium is toxic to freshwater fish and plants. It's a lot more toxic that just leaving the pH at a low value.
So really, if you are asking the question of what parameter matters, then you have to look deeper into what causes the parameters to rise or fall. That is where your answers are, not in absolute values or in absolute stability. If your KH is high because there is a load of Sodium bicarbonate in the tank then you need to lower the level of toxic Sodium. It won't matter about the bicarbonate part because that's not the part that is dangerous.
It's more important to know what the values in the tank are when the tank is healthy. The equilibrium numerical values are not really important. They are more important to understand as a baseline set of values, so that when they change you can then analyze the tank to determine the causes. If the cause is determined to be innocuous then you don't have to worry about what the particular number is or what the stability is.
Some people use substrates or hardscape which contains lime. They add CO2, which produces a weak acid, and this dissolves the lime, so TDS, GH, pH and KH all increase over time. The fish don't really care too much, except that there is still the problem of organic pollution, caused by the fish and plants themselves, which, as I mentioned, also increases the TDS. But what happens is that the hobbyists focuses on the fact that some agent is causing the GH, TDS and PH to all rise, when they really ought to be paying attention to the component of TDS rise caused by the fish and plants themselves. All they have to do is to change the water, keep the tank clean and not worry about anything else, but The Matrix teaches us to spend enormous amounts of energy trying to fix a problem that doesn't actually exist.
Cheers,