For Fish Only- while like you said its never what would be called a good thing, I think safe for fish only is anything under 30ppm for long term. I have seen it spike off the charts for weeks at a time with no casualties but it is certainly not healthy or humane to keep in such conditions.
for Reef or mixed ect tanks I would say keep it under 10ppm. And heavy spikes can be leathal even for short durations. Keep it under 10ppm max.
Trates are the real lessor of the evils so when they are high usually something else was high first and thats where the problems are for lethal consequences.
Sorry if this sounds condasending but why not just run at 0 across the board, Its not hard to do, its usually something overlooked that is CREATING the levels, not build up or inhabitants. IME. Go back through your system peice by peice and find the cause.