The absolute highest current flows when starting the bike. Its path is from the battery positive terminal, through the positive battery cable, through the solenoid, through the starter cable, through the starter, and back through the negative battery lead to the battery negative terminal.
Everything in this path is heavy duty to handle the high current.
First assume the wiring is completely stock with no modifications. If the negative battery cable connection were to fail (corrosion, loose terminal, whatever), then no current would flow. No harm, no foul.
Now suppose another wire were added from the battery negative terminal to the frame. Now if the negative battery cable connection were to fail, the return current path would be entirely through this added wire. If it is a much smaller gauge it may overheat/melt/catch fire/etc.
Things can get worse if the additional wire doesn't attach directly to the frame but goes through the wiring harness to reach the frame ground. The wiring harness may be severely damaged in this case.
So if an extra ground wire is added to the frame it should be large enough to handle the starter current.
My preference is to leave the bike ground wiring unmodified.