Yes, you can just abandon the return flight(s) and not suffer any consequences. However, should there be a significant schedule change, you may be able to call in and get a refund or push the date out for later use without a fee. Also, by cancelling the flight, you may allow someone else to get confirmed on that flight and keep them from sweating it out at the gate. You need to decide your travel karma quota and do what you feel is right.