some sort of unofficial rule
I can think of fifteen or twenty reasons why such a rule would be pointless, in fact I can think of six thousand, four hundred and ninety-seven.
"I went to Spain in nineteen sixty-five" said Contrex. In fourteen hundred and ninety-two, Columbus sailed the ocean blue.
In American usage, various opinions co-exist. For example, the Associated Press Stylebook advises spelling out the numbers one to nine and using figures thereafter, while the Chicago Manual of Style recommends spelling out the numbers one to ninety-nine and using figures thereafter.