It won't mess up your parameters. And it isn't the distilled water that is helping your nitrate go down, it is the water change where you are replacing your 40 ppm water with 0 ppm water (which happens to be salt mixed into distilled water).
With that said, it is much cheaper in the long run to get an RO/DI system of your own. Distilled water is expensive, and you can get a quality RO/DI unit for $200 bucks (BRS or Marine Depot systems work great). Do the math and figure out how many water changes with distilled water it will take for you to break even on that RO/DI unit and the choice becomes clear.