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I think that tagging may be appropriate in particularly obvious cases, or particularly egregious cases involving hundreds of merit points and several posts. But generally you should start out by assuming good faith, and only change that opinion as the evidence really piles up. Tagging someone immediately after an instance of apparently-inexplicable meriting is too trigger-happy IMO. Even if it is a case of illegitimate merit, even hundreds of illegitimate merit points are not much of a problem IMO, so you have to ask whether it's worthwhile to possibly make a mistake by tagging someone who is merely suspicious.