So, I use Teflon saucepans for cooking vegetables, only.
Tomatoes have to be added after the onions are done. Tomatoes have inherent water in them and are obviously below the pan's temperature.
I am afraid that in the long run it may ultimately damage the Teflon cookware.
What's the way out?
Teflon will slowly damage over time no matter how you treat it. Certain things will speed up this damage: too high heat, some metal tools, sandpaper. Unlike an aluminum pan, Teflon will not react to high acid foods like tomatoes by getting damaged. It is a non-reactive material. You will see black flakes start to appear in your food when the original chemical bonding begins to fail on your pan, which should signal that you need to grab another one.
Water content of the vegetables has little or nothing to do with Teflon deterioration. The reason you add wet vegetables after "dry" ones is that the water content of the wet vegatables will prevent the dry ones from searing and carmelizing.
However, more to the point of the question, the way you should fry or saute vegetables in a non-stick pan is:
That being said, I personally use stainless or cast iron if I'm just frying vegetables.
EDIT: I previously posted some incorrect information about acidity and Teflon. I have removed this information based on research. As it turns out, Teflon is highly resistant to acids and would not be damaged by acidic foods in any way -- it can resist hydrochloric acid. Reference: http://www.calpaclab.com/pages/chart.html
Tomatoes specifically will not damage your teflon pan any more than any other cold, wet vegetable. Teflon was used to store a form of uranium in the Manhattan Project because it's so nonreactive. Without it, uranium hexafluoride would eat through storage containers.
It sounds like you were also talking about thermal shock--which you can observe with a hot glass dropped in cold water. That happens on a very, very small scale with pans and food-to-be-heated. I think other factors--like scratching from spatulas and accidental overheating--would wear out the pan far, far sooner.
The primary concern in the precaution against cold water on hot pans, Teflon or otherwise, is typically directed at running cold water from the tap over it (ie when rinsing to cool it down quickly before washing it). Tomatoes and other water bearing ingredients poise a negligible risk to warping relatively.
I regularly add stock and other cold/room temp ingredients and have not experienced warping in 8 years with an inexpensive pan set. how else could you fry an egg or deglaze a pan?
