There are no rules about this any place that I can find, so I can only offer advice. The only purpose of the letter sign (with one exception I'll get to) is to provide clarity between contractions, particularly single-letter whole word contractions and letters that mean letters. In uncontracted braille, there (duh) are no contractions. Think of it as reading print. There are no contractions in print and letters that mean letters are identified visually and by context. That includes lower case Roman numbers. Therefore, no letter signs.
I'm having a brain freeze, no doubt, but I don't get what "m in numbers" refers to.
The exception I mentioned above is letter/number combinations such as 42b. The letter sign would be necessary for the b so that the number is not read as 422.