I don't have TiVo--I have the DVR service that comes with DishNetwork.
I can lock a recording so that it isn't automatically deleted after a set time period, or when I run out of room and it wants to record something new. However, if I forget to lock it, then once I've filled my 100 hours, it starts deleting stuff, starting with the oldest, unlocked recording.
It's worth it, even if there is a database somewhere with a list of what I watch. It's completely changed the way I watch television, giving me complete control over when I watch a program. And I never miss an episode.