Before dental work shoud I take chlordiazepoxide to relax jaw?

Ask dentist. Ask your dentist. He/she is in the best position to counsel you.
Probably not. In general, there is no need to take chlodiazepoxide before seeing your dentist to relax your jaw. Certainly, do not take it or any prescription medicine unless it's prescribed for you by a doctor. Only if your see your dentist and he recommends and prescribes it, should you take chloridiazedpoxide. But generally speaking, no.
Yes. If you have tight muscles or if you grind your teeth, it would be helpful.