For a planted aquarium, are some nitrates preferable over 0 ppm nitrates? If so, what level?
As hoppy and a few others have noted, there is no definitive amount needed so long as you are not running at a deficit ie lets say your accumulating 10ppm nitrate / day, and your plants and biofilters consume 8 ppm per day, you are accumulating 2ppm per day and a simple water change fixes that, of filter maint will cause that to drop off a tad.
If on the other hand you accumulate 10ppm / day but the plants need to consume 8ppm to sustain their growth rate, and your bio filtering is consuming 5 ppm / day, you can see your Nitrate level will fall to zero and the plants will then enter a nitrate deficient state.
Ideally your desired perfect level is ZERO ppm.
just if you get there make sure the plants are not suffering by getting to little of what is left after the BioFilters, or you add N
just watch the plants, they will let you know.
BTW if you need to add N ( nitrogen ie Nitrates ), use KNO3 ( spectracide Stump killer ) dose very very carefully with this. My self i just dose Leaf-zone and Trace, and increased my fish load.
i accumulate 5 ppm per week currently, but evaporation requires 5+ gallons of new water every other week, which dilutes the Nitrate accumulation, but am playing with DIY CO2 so that will drive up the N Uptake from the plants...... it;s all tied in.... aint plants fun