I tend to think that leaving any input pin floating is bad practice. Tie the unused inputs off to ground through some suitable resistors, then you only need to do a zero check on the whole byte rather than check for an individual bit.
If, instead of tieing unused port pins to ground, you just write a 0 to them, you have the exact same effect and you can use the pin with no cut when you discover you need another pin - also simpler layout due to no resistors.

Erik