Even in planted tanks, you want to do water changes. In growth accelerated plant tanks (co2 injection, high light, bi-daily nutrience addition), you want to do large water changes to remove any toxic substances from the water. Generally, if you notice mature leaves dying off quickly, that means your water is a bit toxic. All of my planted tanks get 40-50% water changes once a week.
As for fish only tanks, imho, regular water changes are important. Not only in removing nitrates, but other things that might have built up in your water. Nitrates are harmful in high concentrations, not only that, but they are also an indicator of other things that may be harmful in your tank that are undetectable by normal means. Put it this way, would you rather breath in polluted air, or fresh air? You can survive in both conditions, but you'd do much better in fresh air.
shmeeb001 brings up a good point. When you start doing your water changes, start very slowly. Begin with 5-10% once a week for a few weeks. Bring it up to 10-15% once a week after that for a few weeks, continue to do so until you are changing about 30-40% of the water once a week. Frequent water changes also helps stabilize your tank's chemistry. Many water changes makes your tank's chemistry similar to your tap water. So if for some reason you need to do an emergency large water change (ie. you drop a crap load of food by accident) you can do so w/o much shock to your fish. My fish only tanks get the same amount of water changed as my plant tanks... about 40-50%. I can say with confidence that if I do a very large water change (for example 80%), my fish will not suffer from a drastically changed water chemistry (assuming the water temperature doesn't change).
HTH
-Richer
