Unused MCU pins, better to tie to GND or use software to prevent floating?
There are many things that you can do, but the correct one depends on what you are trying to accomplish.  I'll go over each of the common things:


Leave the pin unconnected, set as an input, and no pullup/pulldown.  This is probably the worst thing you can do.  A floating input could cause input noise, even if that input is not being used for anything.  This could result in increased EMI and increased power consumption.  There is no good reason to do this.
Leave the pin unconnected, set as an input, with internal or external pullups/pulldowns.  This option is nice because the pin is available for future use (in case your design is not proven to work).  Accidentally shorting the pin will likely not cause problems.  This solution does nothing for EMI, however.
Leave the pin unconnected, set as an output, and driven high or low.  This is nice because the pin is still available for future use, but accidental shorts could easily kill the thing.  This is the worst case scenario for EMI, because each pin forms a tiny stub of an antenna.
Set the pin as a input, connect the pin to GND or VCC.  The pin is not easily available for use if you have to rework the design.  A good solution solution for EMI.  One nice side benefit of this is that you can sometimes more easily route your power/gnd on the PCB since you can use the pads as a "route through"-- especally helpful on 2 layer PCB's with QFN or TQFP packages.
Set the pin as an OUTPUT, connect the pin to GND , drive the pin low as appropriate.  Sometimes this is called a "virtual gnd", and improves the grounding of the chip.  This solution is the absolute best for EMI, but does not tolerate mistakes of any kind.  For example, if you enable the output before setting the output value low and the pin is driven high for a moment then you risk damaging the part.  This is almost never done in MCU's, but is more frequently used in FPGAs where additional grounding helps to minimize simultaneous-switching-noise.  I would not do this without consulting the chip manufacturer.


Without knowing specific issues that you might be having, solution #2 or #3 are the best balance of pro's and con's.  Solutions #4 and #5 are reasonable solutions if EMI is a problem but your design is otherwise close to final (I.E., you are not likely to have a design change that will require more pins).