Assuming you're in at least 232 gear plus i would go for crit/str gems in every yellow socket assuming the socket bonus is strength and it doesnt include a blue gem to activate. (This is if you use nightmare tear and agi meta).
Whichever piece of gear has a single blue socket, either alone or with other colour sockets, that has the largest amount of str as a socket bonus, is the one you place the nightmare tear in to activate.
As unholy it seems the latest best dps gemming is to go str/haste instead at higher level gear instead of crit/str.