Even though the test kits show acceptable levels, it's always good to maintain a regular water change regime. Don't wait for something to go wrong before you fix it. High nitrate levels have been proven to stunt fish growth, so unless you have lots of plants getting rid of the nitrate, it's not good to have it building up in the water. Also, remember that aquariums are CLOSED ENVIRONMENTS. In the wild, fish swim in waters that are constantly being changed due to water movement or rainfall. In an aquarium, it's the same water being circulated until you replace it.
Think about it this way: if you sat in your car with the doors/ windows closed, you would have enough oxygen to survive for a while before you have to let in more fresh air. Just because you CAN survive does not mean that you will THRIVE on that stale, recycled air.
On a final note, fish are very adaptable. If you ignore your water changes, fish CAN adapt to bad water conditions without dying, as long as the change is gradual. When you finally get around to doing it, you figure that you should make up for lost time and do a BIG water change - that's what's likely to kill your fish. Even though you're adding fresh water, the sudden changes in parameters will send your fish into shock and make them more susceptible to sickness/ death.
Just remember that we should always strive to give our pets the best possible conditions rather than simply letting their well-being be dictated by our schedules.