I tend to paint deep cuts with khorne red, put a bit of agrax earthshade in there and then add bftbg, the darker colour gives an impression of deepness although I'm sure BaMs method works too, also maybe add a small bit of reddening around the wound, the skin tends to go more pinkish or red around a cut, there's a medical reason for it but can't remember exactly, something to do with blood rushing the area to form a scab maybe? Anyways you wouldn't need much, just a bit of a glaze next to the edges