Not connected pins on microcontrollers: what are the advantages/disadvantages of internal pull-up versus tristate?
When you have a not connected pin, the first thing you need is to clamp it to a rail, either Vcc or ground. You need this because a floating state may cause an intermediate state in the input buffers, leading to static power absorption.

To force a level, you can either set the pin as output or input with internal pull-up.  My measurements didn't show any difference in the power absorbed with the two solutions, but the manual (of a Jennic microcontroller) suggested to use the latter (input with pull-up). I'd say it's reasonable, because it uses a weaker pull-up than the output mode, and this may reduce leakage.