I knew that both forms were common because I was lucky enough to have a Latin teacher who assigned as extra credit "write this number in Roman numerals" and als

I knew that both forms were common because I was lucky enough to have a Latin teacher who assigned as extra credit "write this number in Roman numerals" and also accepted additive notation for this reason. He even argued that IIIIIIIIII could be a valid if cheesy alternative to X based on very early texts, but this might have just been a bluff. I never tried to write 123 as IIIII...III.