Those are showing right at the edges of the texture. The face texture map ends right in front of the ears like that. And you say this only affects some textures. I think the texture map just isn't painted all the way to the edge. The solution would be to extend the edges of the face texture, like hborre said.

The lines we used to get from not picking unimesh were cleaner than this, I don't think it's that. (I think it's still a good idea to pick unimesh skinning, I just don't think that's the cause of this specific issue.)