74HC/HCT: What to do with unused inputs and why?
There are a few considerations that have not been mentioned in other answers.


Sometimes the unused input plays an important role in the logic of the part. An example would be a 4-input gate where only 3 inputs are actually used. In this case the logic level that you tie the unused input to must be selected properly or else the logic function of the used functions will not work.
In some business / industrial segments it is necessary to test all functions in each part on the board even if they are not used. This is done to ensure that some nascent fault in a chip does not expose to a higher chance of catastrophic failure of the part. The addition of pull ups or pull downs on each unused pins allows automated test equipment to toggle the pins which would not be possible if they were hard tied to VDD or GND.
There are cases where it is handy to keep unused gates available for possible future rework to tweak the design in the case of bugs found, need to invert or combine signals or other things. Pins hard tied to VDD and GND are very much harder to rework so added pull ups or pull downs provide access pads for the rework.