Final answer:
Roman Numerals are used in ionic naming for cations with variable charges, but not for cations outside the 'Danger Zone'. The statement is true.
Step-by-step explanation:
In ionic naming, Roman Numerals are used for cations with variable charges, especially for transition metals. These Roman Numerals indicate the amount of positive charge on the cation.
However, if the cation is not in the 'Danger Zone' or does not have variable charges, Roman Numerals are not used. Therefore, the statement that you must not use Roman Numerals in ionic naming UNLESS the cation is in the Danger Zone is True.