Plastic or Metal makes no difference really. What is important is the grounding is correct. Use a Star ground system so all shields and grounds go back to one point and you won't have any issues. Also, it's good wiring practise to route any signal wires as far away from high voltage wires as possible. Obviously it's not always possible to do this so some signal and high voltage wires will run side by side, in this case, don't tie wrap them together.