In a drawing - or in a model? If it's a drawing, just create your own suffix and hide the unit string.
To be technically correct however, under the SI units definition, "milimeter" is abbreviated to "mm" not "MM". The first letter prefix "m" in lowercase means milli or 1/1000th of [unit of measure]. "M" however means "mega", or 1000,000 times [unit of measure]. So by writing "MM" to represent millimeters, you might end up with something rather larger than you expected. (Not sure if "M" is even a recognised unit of distance?)
Good example: 1Mm³ means one cubic megametre, or the size of one cube of 1000000m by 1000000m by 1000000m
Not the 1mm³ you might have got if using the correct case on the unit string.
Like someone said before, don´t mess with units...
You will produce documentation with bad information...
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