Nil Nitrate is Bad in PlantedTank?

Hmmm. I'm beginning to think I have misunderstood the role of nitrate in an
established planted freshwater aquarium. My ammonia level is almost always
zero (or so it appears from my Aquarium Pharmaceuticals test kit.) But
recently I have noticed newsgroup responses indicating that some nitrate is
good for the plants.

Maybe my weekly 20% water change is excessive. I'd appreciate
advice--should I reduce the weekly quantity, reduce the frequency, or just
stop being so damn obsessive? My plant growth is disappointing. The other
paramaters are normally: pH 7.6, GH 6, KH 5, PO4 0.25. (No CO2 injection,
quite heavily stocked 75gal tank with about 25% plant cover).