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? 
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. 