From the following excerpt from "Media Manager System Admin Guide for Netbackup 5.1"
"....
Return Media to the Scratch Pool
RETURN_UNASSIGNED_MEDIA_TO_SCRATCH_POOL = YES|NO
This entry is read and interpreted on hosts where vmd is running.
If RETURN_UNASSIGNED_MEDIA_TO_SCRATCH_POOL = YES is specified or if no entry is specified in vm.conf, Media Manager returns expired and unassigned media (media that was originally from the same scratch pool) to the scratch volume pool automatically.
If RETURN_UNASSIGNED_MEDIA_TO_SCRATCH_POOL = NO is specified, the automatic behavior of returning media to the scratch pool is disabled and must be done using one of the Media Manager administration interfaces.
...."
I would say that the answer to your question would be (or should be) YES, but it should be easy enough to test.