What’s a good backup media rotation?

Our company’s current tape rotation scheme consists of full backups every night, and every other friday backup going offsite. Full daily backups are fine because we can fit all of our data onto two DLTs, and it runs at night when our offices are empty. However, I’m not sure that this is the best scheme. It is expensive to retrieve our offsite tapes if we need to do a restore. I’m looking for any suggestions. Our office has about 150 people, and is a 9-5 shop.