The subtractive form of Roman numerals seems to have been used relatively infrequently in ancient and medieval times, only becoming the standard recently. It was used mostly to save space, and so perhaps appears on carvings more than it would otherwise. The rules were somewhat lax, so you see things like "XIIX" for 18, on the tombstone of a centurion of the 18th legion). And, when doing addition or subtraction, you'd have to convert the subtractive form to the additive first.
Florian Cajori, A History of Mathematical Notations
You would probably be OK since VIIII was used on official Roman coinage. In the Marc Antony legionary coins both VIIII and IX were used as 9th legion denominations.
