Identifying bodies from dental records is Truth in Television, but it works a lot better in fiction than in real life. Many people in Real Life do not go to the dentist regularly, if at all — this is a special problem with the homeless, who are also more likely to become murder victims than the average person.
In practice it is not always easy to find out who a deceased person's dentist was if they have no immediate family to ask. Then you have the people who wear dentures — most denturists do not keep records even for seven years, and the poor can wear the same dentures for twenty years or longer. Worst of all is when the body is not found anywhere near where the victim lived; this was a near-insurmountable roadblock to identification before the development of DNA databases such as CODIS note Even with these resources, identification can still be a problem today if the body crosses a national boundary — say, if it is dumped into the water in upstate New York and washes up on the coast of Newfoundland. And even if the body is found near where the victim lived, it still needs to have a head.
Nevertheless, dental records are important identification tools, and not just for the deceased. There are hundreds of amnesiacs living in mental health facilities in the US and Canada (mainly dementia patients and head injury victims) whose identities are unknown. When these people are identified it's usually through dental records.