When should a post be flagged?

  Please don't complain that a submission is inappropriate. If a story is spam or off-topic, flag it. Don't feed egregious comments by replying; flag them instead. If you flag, please don't also comment that you did.

  Please don't complain about tangential annoyances—e.g. article or website formats, name collisions, or back-button breakage. They're too common to be interesting.

Others seem to flag things they disagree with.

I probably could be convinced to flag boring submissions that result in the same flame wars each time.

For really bad ones, I also email.

That's kind of the standards I use. But there's no hard-and-fast rule (that I recall) about when and how. We're allowed some discernment and judgment.