If the suspicion is on technical matters, don't flag. Otherwise, use your flags gently but judiciously. Keep a look in your profile and try to understand which flags are rejected, and which are helpful. If your flags are helpful, read that as an approval and continue with your flagging pattern, whatever it be.
When I haven't enough reputation for casting close votes, one of my favorites was flagging questions as exact duplicate. If the question is not an exact duplicate, I think it is better not to flag it, because assesing the validity of your flag probably requires some knowledge on that tag.
Now, most of my flags are for not an answer. I don't think there is a borderline for that. If the answer is technically wrong, it is an answer: downvote and don't flag. If the answer is a comment, another question, or a thanks/me too note, then the flag is valid (you can downvote in this case, and advanced users will notice the downvote and remove the post, but new users may not take that action, hence I flag in order to help cleaning the site).