When two or more devices are connected to a common ground through different paths, a ground loop occurs. To prevent ground loops, all signal grounds need to go to one common point and when two grounding points cannot be avoided, one side must isolate the signal and grounds from the other.
>> I was once considering doing a common ground point on all my guitars, and then I read an article pointing out that there is always a common end at the output jack/cable.
The real problem is when plugging into 2 separate amps with both grounded, or effects etc that get power sources from different outlets...this is where 2 prong adapters help (on all but 1 plug). But the guitar circuit may be a smaller version of the same issue, if the outlets have a common ground point in the building wiring. Certainly wont hurt and its easy to do that way.
" Garcia's Guitars were not every other guitar . His guitars use a 10 AWG wire ground buss from ground lug on the vol pot to the ground lug on the output Jack. This allows for all components, body, tailpiece and p-up cavity to ground at a a single ground point to avoid ground loop noise.
>> This could be confusing to some I think. The wire attached to the lug provides the ground link to vol pot, which is practically the same as attaching it to the back of the pot. Here the back of the pot will have continuity from either the shielding or a direct connection from the blade/lug...e.g. they are often bent back to touch the back of pot on Fender Strats. Im not sure what affect not grounding the pot would have, especially with shielding...never tried it; and I certainly don't know about Garcia's actual guitar.
If the shielding isn't grounded I believe it acts like an antanae for noise! Not good. Shielding often connects to the pots by default as the pot secures against it. If there is removeable pickguard design more care is needed to ensure the cavity side shielding has continuity.
If you're like me its hard to know how important some of these things are because I've never tried anything but the typical wiring models. Only when I have a problem do I go in and see what is wrong.