While I would agree with rocketgeek about unused pins in a strictly engineering context I think for what we are doing here it is a little bit of an overkill. Sure a floating input will pick up noise and transfer that noise into the rest of the processor's circuitry but in the context of what we are doing then it is not going to affect the performance of a system. It is certainly not going to make the difference between something working and not working. It might affect long term reliability, that is once a year it might make an application malfunction once but maybe not even that often.
In discrete logic gates then floating unused pins can affect the propagation delay in a gate but we are not dependent on those parameters here, that can be left to high speed systems. Most of what we do is restricted to 16MHz and nowadays this is not considered a particularly high frequency.
So my take is yes but no. However, in this case where the pins were actually doing something defiantly they should not be left floating.