The Answer:

According to librarians at the National Institute of Standards and Technology, while MIM is more convenient, MCMXCIX is favored because of earlier precedents with numbers such as 49 (written as XLIX rather than IL); however, the librarians point out that purists use neither MIM nor MCMXCIX, opting instead for MCMXCVIIII. The ancient Romans, they explain, did not use the 20th century convention of IX for the number nine.

—The Editors