What is the difference between pulling up multiple digital IC pins to Vdd with a single resistor vs. one resister per pin?
Assuming DC loading isn't a factor here (generally not with modern CMOS devices), one advantage of the first (individual resistors) is that it makes it easier to use one of the unused inputs just by connecting a wire/signal to that pin.  If the second option were being used, you'd have to isolate the desired pin from the shared pullup trace first.

Another reason for individual resistors is that it makes routing easier on the PWB.

Many times you'll see a hybrid, where you might have one pull up resistor for a given IC.  Or one resistor for a common signal, such as an enable for a bunch of differential line drivers.