Good question - someone may be able to answer better than me. I do know nitrites can be toxic to land plants so I suspect they same could be true to aquatic plants. However, the toxicity is usually in the root zone, and I would not expect there to be much nitrite in the root zone in an aquarium.
I bet Diana Walstad's Ecology of Planted Aquarium would shed some more light on this. I'll see what I can find.