Which of the ground returns you use doesn't matter, but the point is to use one ground return for each power line - the point I'm trying to make is that it's not only important to properly supply power to a device, you'll also have to supply a clean ground return.
There's only so much current you can run through a wire, depending on diameter/cross section (and material). If the line is too weak/thin it'll cause a voltage drop which will in turn cause all kinds of problems.
But you can't use arbitrarily thick wires, so you use a whole bunch of them. Bridging them together to save lines doesn't make sense in that way.
I once repaired a PET that had been sitting in storage for 15 years - after fitting a 32K memory expansion it just wasn't working properly any more. The main problem were the power supply cables that were getting quite warm,, so I replaced them with the thickest I could find. Tweaking the power supply from 4.8 to 5.2V did the rest and the PET worked like a charm.
