Todd is just wrong about his version of the ground mods.
The whole problem is that Gottlieb used a daisy chain of connectors to bring ground from the power supply, to the CPU, to the driver board. By the time ground gets to the driver board, if the connectors aren't perfect, ground could actually not be "0 volts". Due to the resistance of the connectors, it could be slightly higher.
The problem with this is due to digital logic... it doesn't do well when things aren't perfect "0" or "+5" volts (which translates to "0" or "1" in it's digital mind.) What if the voltage is slight above 0 volts? Is that a "1" or a "0"? It gets confused, and wacky stuff happens (like locked on coils.)
To make things obvious to the digital circuit, it's best to not rely on connectors, which with time, can gain resistance. That's why you've seen the ground modifications to the power supply, CPU board, and driver board on system80 games. To be frank, the rest of the boards don't really matter in regards to ground. But if you want to tie hard grounds to them too, it certainly isn't going to hurt anything. (the sound or light chaser boards sometimes need better grounds, but nothing smokes and burns if that doesn't happen.)
So bottom line, on sys80 and sys80a games....
0. If there's an orange 12 volt filter cap on the bottom board, replace it with a new 10,000 mfd 20 volt cap. Time: 2 minutes.
1. Take the main green ground wire that attaches to the lock in the backbox, and bolt it to the power supply frame. This involves undoing two screws. Note this wire goes down to the bottom board's brass metal "ground central", where all the green wires connect together. Time: 10 seconds.
2. Modify the power supply supply with a 3" piece of wire. Run it from the power supply negative lead of cap C1 to the metal power supply ground frame. Time: 1 minute.
3. Run a 6" piece of wire from the CPU board negative lead of cap C1 to the metal frame of the power supply. Time: 1 minute.
4. Run a 18" piece of wire from the ground plane of the driver board to the metal frame of the power supply. Time: 1 minute. Check out a picture to make sure you do this step right. It's a bit tricky to find the right trace.
5. Optional: Double up the +5 and ground lines on the double sided connector going from the CPU to driver board. Note this is the modification Todd does. Frankly it's helpful but not mandatory if steps 1-4 were done above. But go ahead and do it, as what the heck, it's 6" of wire and four connector pins. Time: 2 minutes
6. Don't listen to Todd anymore in regards to Gottlieb grounds.
So in 10 minutes you can do the job RIGHT. I hope this helps.