If you're writing short sessions, you might want to just use CD-R's. Remember anytime you go to add a session, you stand a chance of messing up the disk.
Although I do understand the space savings of fewer DVD's, I also understand the reality of adding data to an existing piece of optical media as well. And for some things you may just not want to take that risk. Like backups. (Unless you alternate the media so you're not adding to your most recent backup.)