In my humble opinion you are free to use the shadow stone once you've reached 50% crit chance (without buffs).
Until this point the thief could be an option but I don't like it that much and prefer the warrior stone instead.
My main is a nightblade and I'm around 50 - 60% cit chance. I switch between shadow and warrior a lot. But with all the passives I think the shadow stone wins (at higher crit rates of course).