**Sharps and flats can be added to specific notes, and as Stu has pointed out, any sharp or flat in the key signature can be nullified by adding a natural sign before the note in question.**

Taking C minor scale as an example, it would be easier to assign a key signature of two flats (E and A) rather than three because modern music demands these scales have chromatic capability. Unless you are playing in true Aeolian mode then three flats would be acceptable.

One advantage of this is that minor scales would have their own specific key signatures apart from major scales instead of borrowing the same ones.