A "Ghost Ring" is nothing more than a larger aperture. So, your real question is "What diameter aperture should I use?" The only way to know is to try them out. A smaller aperture increases depth of field (which sharpens focus), but reduces how much light passes through. A larger aperture increase the amount of light that passes through, but decreases depth of field. The ideal situation would be a small aperture and a bright sunny day- increased depth of field and more than enough light. How often do we shoot in ideal conditions?
For myself, I find that the small National Match aperture of my M14 is too small unless it's a bright sunny day. Overall, I prefer larger apertures because I can see through them even when the light is less than ideal. It's easier to be precise with smaller apertures, but larger apertures afford a faster coarse sight picture. With consistent cheekweld and good form, precision loss with a large aperture is minimized.