Can SS pin be tied low if only one SPI device is used?
[ @Kevin and @akohlsmith should post their comments as answers.
This is to expand on their comments. ]

SPI1 doesn't prescribe the exact behavior of the SS (slave select line2).  This behavior is dependent on the implementation of each individual device.  I have seen  devices that can operate with the SS permanently aserted.  I have also seen devices that require falling and rising edges on the SS.

Although it doesn't say that explicitly that the SS must be toggled,
the design guide (p.31) says that SS starts and ends the SPI transaction.  It would be prudent to assume that SS has to be toggled for the SPI transaction to come through.  

1 SPI is more of a custom, rather than a standard.
2 CS (chip select) is another name for slave select.