I guess some can keep them completely at zero, I really never have been able too, but as long as they are low, it has always been fine for my tanks, my clam likes them, in fact a little is required to keep my clam happy, as do my soft corals in a mixed reef tank, it's bit of a trade off...
Here a good read...
Reef Aquarium Water Parameters by Randy Holmes-Farley - Reefkeeping.com
The below is a post & reply to Randy I recently read:
Q: I can't seem to get my nitrates to below 5 ppm
A: There is little evidence that nitrate appreciably harms corals, and many reef tanks have higher nitrate than you without apparent problem.
In a RSM with a carnivore Trigger fish ... it's going to be hard to keep the nitrates at zero, triggers can really eat & poop a lot. Triggers are messy eaters. The Key going to just be keeping up water changes & fresh chemical filtration media - imo - I would not worry about low levels.