Not all IO pin architectures are the same. In fact, they are usually more different than one would expect. Usually either High-Z(floating) or Open Drain Low will yield the lowest power consumption, as documented in the datasheet or references for the part. On some devices not even all the IOs will have the same architecture so it is even possible that some unused pins should be left floating while others should be driven low or even configured for use by a certain peripheral for minimum consumption.
In some applications, other parameters may matter more than low power consumption, like ESD tolerance or handling ruggedness. Again, the answers may be different part to part and pin to pin and requirements should be considered.
In short, best not to make assumptions about black box systems. Look it up, take measurements, or contact a support engineer.