If the auxiliary I2C pins are not used are they grounded or pulled up on the MPU9150?
If the auxiliary I2C bus isn't used, the pins should be left floating.

Invensense doesn't say this in the Spec Sheet.  I had to find it in their docs for the Evaluation Board.  If you download the Eval Board User's Guide, and look at the schematic in section 4, you will see that both of these pins are pulled high.  But, the component value is "NM", for "not mounted".  They are leaving the option open, just in case somebody wants to hook another device to the Eval Board.

I often find that docs for Eval Boards or Dev Kits give better usage information than Datasheets.  Not just with this manufacturer, either :)

Good luck!