I think NFPA 99 requires it, I don't know if you have to follow it.

You can find more info in these links, keeping in mind they are trying to sell their product.
http://www.leviton.com/sections/prodinfo/receptic/sheets/tensiontester.htm
http://www.leviton.com/pdfs/tensiontest2.pdf

Bob Badger
Construction & Maintenance Electrician
Massachusetts