I've never used Scupltris but I have used Maya. When you select the head in Poser it will have a positive Y value. What I'm thinking is that somewhere, either in the export from Poser or the import into Sculptris or even when you're sending the model back to Poser from Sculptris the Y value is being altered. I can't remember exactly but I'll see if I can get the chance tonight to try out what I think is wrong and let you know.