By industry standard, both ISO ad ASME (ANSI), inch units are expressed with trailing zeros of same significant figures but not leading zeros.
And, ISO and ASME specifies that SI units, or mm are to be expressed with leading zeros but not trailing zeros nor is significant number of figures to maintained.
Therefore, one cannot use the matching significant figure method to express default tolerances when using SI units.
You may need to study up on ASME, ANSI and ISO dimensioning and tolerancing standards.