To backup the SMF files I doubt about using full or incremental backup scheme.
The incremental backup could be from Monday to Saturday taking as reference the Sunday full backup:
http://www.cyberciti.biz/tips/how-to-backup-mysql-databases-web-server-files-to-a-ftp-server-automatically.html
I´ve read somewhere about people having problems when extracting or having bugs with some GNU TAR versinos.
Thnaks a lot.