Hound - thats not really true. If done correctly, perhaps the only stressful part of a water change is having a person constantly hanging around the tank fiddling with things... and if you're vacumming gravel, perhaps having a tube moving throughout the tank. However, if you're only doing a water change, then you can probably think of ways to minimize yourself from hanging around the tank. Just make sure that the water going into the tank is simliar in temperature, and pH, and your fish will be fine. I do two 50% water changes in a row once every 1-2 months, and I do 50% water changes once a week, and all of my fish thrive. The zebras in my Malawi tank have bred countless amount of times (though I've never bothered trying to catch the fry in that tank, so they end up getting eaten), and even the tetras in my planted tank have bred several times.
In a nutshell, the more often you do a water change, the more similiar your tank's water will be compared to your tap water. So in an emergency, you can easily do a 100% water change without much fear.
HTH
-Richer