I think it depends on the part and the other adjacent panel lines. Where the part of the real thing has no panel lines, such as joints across a canopy or fabric covering or where there is no original panel line (like a spinner), you would cut off all the black outline. I think your scan shows rivets, so I would try to match the printed non-cutting panel lines. If the joint is a butt joint, I would cut each part leaving black one half the width of printed panel lines - most of the time the joint would itself would show that much so very little black would be needed. If it is an overlap, like fillets, I would leave black as wide as the adjacent printed non-cutting panel lines.
After all, it is only paper and make sure to make copies so that you can try a couple different ways if you aren't sure. In the end, whatever pleases your eye works the best and have fun
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Regards,
John