The bottom line is yes. You do need to keep up on your water changes no matter what your test kit tells you. It is important to test your aquarium water, but you cannot assume everything is perfect just because the various chemical levels test normal.
Test Kits Don't Tell You Everything
You can measure a number of water parameters in a saltwater aquarium. But you cannot measure every single biological relevant reaction going on in aquarium with commercially available test kits. Water changes are insurance against problems that haven't shown up on your tests yet. Water pH tends to slowly dip over time due to organic compounds in the water. These compounds can build up to dangerous levels before causing changes in pH, though.
Other Benefits
Additionally, water changes don't just fix problems with water chemistry. Even in a healthy aquarium, detritus tends to build up on the substrate, giving the tank a dingy, unclean look. During a water change, you can use a specialized siphon called a gravel vacuum to siphon water and detritus from the aquarium. This allows you to clean your aquarium sand or gravel, making the tank look clean independent of water chemistry. Additionally, using a gravel vacuum instead of a regular siphon allows you to clean the substrate and do your water change at the same time.
How Much
At bare minimum, you should preform water changes at least once a month in an established aquarium. In newer aquariums, you should perform water changes whenever ammonia or nitrite reach detectable levels. In aquariums of less than 30 gallons' capacity, you may need to perform water changes more often, since water quality tends to degrade more quickly in such tanks. When you perform a water change, you should replace roughly 10 percent of the tank's water volume.
How to Perform a Water Change
When you perform a water change, use either a gravel vacuum or stir the substrate to liberate detritus before siphoning the water. Remove your 10 percent of the water and discard it. After this, mix fresh saltwater. To create saltwater for a marine aquarium, mix commercial salt mix with reverse osmosis water. RO water has been purified through a process that removes all dissolved minerals. Make sure the new water has the same temperature and salinity as the old water to avoid shocking your fish. Using a thermometer, make sure your new saltwater is within a few degrees of the aquarium water. If it's too hot, let it sit to cool down, since saltwater aquarium water's usually warmer than room temperature. If it's too cold, use a submersible aquarium heater to raise the temperature. Once you have the temperature right, take a reading with your hydrometer to make sure the salinity is correct. If it's low, add more salt and wait an hour. If it's too high, dilute with more RO water.