| Pluck where the sound suits your playing.
Near bridge the tone tends have more middle & treble and plucking above the highest frets gives a nice round tone (not so much of overtones).
Honestly for metal, I suggest closer to the neck for a couple of reasons. First, strings feel looser there so it's a bit easier. Second, you can do faster licks with less digging of the strings. And third, you will probably be using some sort of effects for metal (distortion & compression) so having more overtones isn't necessary since distortion already brings 'em up front.
With a pick, I'd probably be closer to the bridge pickup.
But mainly, it's about the tone you're after. There's no rule, just what feels right.
Also when you pluck you're supposed to pluck along the bass but some people in the metal scene tend to bang the strings (like with slap) to the fretboard to produce some clank which cuts through the mix a bit easier, especially with heavily distorted guitars.
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