Not being as technical as you all, here's my 2 cents: I prefer to add a sheen effect using a second layer as I have the *feeling* that the sheen given by the PrincipledBSDF node is given at the cost of the resulting diffuse color, whereas a second layer, for the sheen, adds the sheen while keeping the color intact.

This may not be a technically correct response, that's the result of my experiments with human skin.

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