I'm pretty sure you do need to cancel any positive space you don't intend to use.
Call the Jumpseat desk through the VRU. Choose the jumpseat option 3, then choose book positive space travel option 4. They can change or cancel your positive space reservation. You could do it all yourself on travelnet, except I can't figure out how to cancel the reservations they make for you. The phone system can be a pain (jumpseat option to book positive space?!), but I love that we can book our own positive space travel to and from training.