Decoupling Capacitor – Multiple Vss pins

I was the one that asked that question. Here is my rudimentary understanding:

You attach capacitors across \$V_{CC}\$/GND to try to keep the voltage more constant. Under a DC circuit, a capacitor acts as an open circuit so there is no problem with shorting there. As your device is powered up (\$V_{CC}\$=5V), the capacitor is charged to capacity and waits until there is a change in the voltage between \$V_{CC}\$ and GND (\$V_{CC}\$=4.5V). At this point, the capacitor will discharge to try to bring the voltage back to the level of charge inside the capacitor (5V). This is called "smoothing" (or at least that is what I call it) because the change in voltage will be less pronounced.

Ultimately, the voltage will not ever return to 5V through a capacitor, rather the capacitor will discharge until the charge inside it is equal to the supply voltage (to an equilibrium). A similar mechanism is responsible for smoothing if \$V_{CC}\$ increases too far beyond its average (\$V_{CC}\$=5.5V perhaps).

As for why you need them, they are very important in high-speed digital and analog circuits. I can't imagine you would need one for an SN74195, but it can't hurt!

I believe the general rule is 1 cap per Vdd pin.

Are you using a ground plane? If so, don't bother attaching the caps to Vss pins. However, if you are using a ground bus instead, then yeah you should attach the cathode of the caps directly to Vss.