The report feature exists for the posts that break the rules, whether they are in a flamewar or not.
It doesn't exist to silence things you think will start a flamewar, unless the post in question breaks any of the rules I stated. Only then should you use the report feature.
Like someone else said, someone could say "I think death note sucks". That might start a flamewar. But does it break the rules? No. So don't report it.
Someone could say "I think trump is a shitty president" or "I think trump is the best president ever". Might start a flamewar. But does it break rules? No. What does break rules is if someone else comes to that person and goes "ur a fuckin idiot how fucking dare u u piece of shit you aren't worth the piss my dog left on the carpet". Then you report THAT person.
If someone says "All black people need to die" or something however, that is something that breaks the rules and will cause a flamewar.
You see the difference? The deciding factor isn't "will this cause a flamewar", but rather "does it break the rules".