personally, i saturate my grey washes and approach the technique no differently than color. if you know how to get solid color without scabbing it should be no different with greywash--in fact, easier because greywash is less hard on the skin. i use 5 to 6 shades and usually layer over multiple sittings to build up that rich, saturated look in the darker areas (50% and darker).
keep in mind i do about one greywash piece a year so im not the leading expert. speaking of which ,the only greywash piece i've done in a long time will be finished up in the next month or two so look out for it in my skin gallery, i'll post it when it's healed since dude is local.