As kibwen said, there wasn't a clear "standardized" form. Most rules were invented much later. "IC" was perfectly understood by Romans that would have used "MXMII" without a second thought.

The reason of "IIII" is of usability for clocks that can be seen from different angles. Six can only be written as "VI" so "IV" is changed to "IIII" to prevent confusion.

Of course there are all kind of urban legends and fake stories of kings requesting the number be written this or that way.

In case someone doesn't know, a fun fact: "I" is one finger, "V" represent the open hand (think pinky and thumb in angle) and "X" both open hands united. So 1, 5, 10.