You don't need a star 0v if you're using optos - since you are separating the logic and motor supplies. As to why you need the optos is beyond me - its just adding more complication. If you look after the 0v wiring correctly, noise into the logic should not be an issue. As to using the 4 0v pins on the L293 as your star point - that should be ok, just make sure the 4 pins are connected together. As another respondent mentioned, these are for heatsinking.