Wolftooth Necklace:
One who has triumphed in one of the contests of might held at the Fang may be awarded a wolftooth necklace in recognition of his martial excellence and athletic prowess. In close combat, a model with a wolftooth necklace always hits on the roll of 3+ regardless of comparative Weapon Skills."
Based on this, I am lead to believe that the Wolftooth Necklace does not allow for a hit on 3+ with any sort of grenade due to the use of a grenade not comparing Weapon Skills.
Edit: The errata changes the text to:
Page 62, Wolftooth Necklace, replace the last sentence with: Against models with a WS value, a model with a wolftooth necklace always hits in close combat on the roll of a 3+.
Just like Murph said in his post. Based on this, I believe even the grenades would hit on a 3+ and reroll misses.