Warhammer 40K 7ed: Haemonculus Covens "It Will not Die" rule effect on units with 1 wound
Nope, IWND is absolutely useless on any model with only one wound...and thus, wracks do not benefit from it in the slightest.

The only models which will really benefit from IWND are characters and Grotesques, since they have multiple wounds each.  In practice, however, Grotesques are the primary benefactors of the ability, since it is pretty easy to Instant Death a T3 or T4 character.