xtrabackup prepare - after dump or before restore?
The chief reason to use --apply-log until the time of restore is to support incremental backups.  

If you make incremental backups, you would use --apply-log --redo-only each incremental backup, and then just before you restore, you do a final prepare with --apply-log.

Otherwise if you don't make incremental backups, there's no reason not to do --apply-log right after you make the back up.  Then it's ready to restore on a moment's notice.