One more question — is it OK to use a defragmenter, e.g., Perfect Disk or Diskeeper, on a compressed drive? Well two more — is there any reason to?
Unless you keep on deleting and adding new files more or less at random on your compressed drive, defragmentation should be superfluous, unless there's something special about compressed drives that make disk utilization even worse than what's usual with NTFS.
But you can check that easily enough by running the Disk Defragmenter.
Basically, defragmentation should only worry you for disks that you need decent performance from. A backup drive hardly fits that description; you won't be accessing it often.