You haven't given quite enough information for anyone to make useful suggestions. You need to analyze the criticality of the data you are backing up, and the importance of transparency in recovering the data. Because you are limited in your backup capacity, you are using a combination of alternating full and incremental backups. With only six tapes, it would be easily possible for the operator to misjuggle the tapes and make your backups unavailable when you need it the most.
I suggest you look at http://www.mikerubel.org/computers/rsync_snapshots/
for a recipe on how to use an inexpensive HDD to make multiple transparent snapshots of your backed up directories. You could then use your tapes to make alternating full archival backups for use in case of a disaster that destroyed both your data drive and your backup drive.