What precautions should be taken while dropping cold vegetables in a hot Teflon cookware?
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.