Everything I am reading indicates the conversion to APFS takes place as part of the High Sierra installation and is NOT OPTIONAL.

For you CCC fans here is another interesting tid-bit about APFS.
Originally Posted By: CCC High Sierra update
  • The Dynamic Linker Shared Cache appears to be broken. This version of CCC will avoid rebuilding that cache if running on High Sierra to avoid system instability.
  • APFS startup volumes can be cloned to an HFS+ destination, resulting in a bootable backup on the destination. The sematics for the Recovery volume have changed, however, so CCC will not create a Recovery volume on the HFS+ destination when cloning from an APFS source volume.
  • APFS startup volumes have new, proprietary Recovery, Virtual Memory, and Preboot volumes. Adding support for these new volume types will take a considerable amount of engineering effort, so support for these is not currently available in this release of CCC. CCC can make a backup of your user and system files to an APFS volume, but it will not produce a bootable backup on an APFS destination. Use an HFS+ destination for now if you require a bootable backup.

{emphasis added}

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein