I dint believe it is in the code, however installation must be to manufacturers specification. It may be in the installation Manual. I know I read it somewhere before but it may have just been a recommendation.
I agree with Iwire, there has never been a code that required it, it was always just a common practice. The only exception would be if the manufacturer specs it then it must be installed as such. I have never seen an installation that wasn't FMC, but I suppose there is one out there somewhere.