It's against the site guidelines to talk about having flagged a post. That's probably why you got told off.

  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.

I only flag obvious spam and malicious links.

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.

A post should be flagged if in a foreign language, spam, or abuse. If it's wrong, it should just be downvoted. If it's off-topic, it maybe should be downvoted. If it's deliberately off-topic, maybe it should be flagged.

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.

There is a "Guidelines" link and also a "FAQ" link at the bottom of the page that should cover most questions that you might have.

I wasn't aware that anyone except maybe the HN admins could tell who flagged a post.