Is this rule of 7 really necessary? Without need for mental arithmetic, if a piece is in Db (5 flats) or D# (five sharps) simply forget the flats and sharps that do not appeal and play the music as if in D.   Same goes for any key. Generally, composers for string instruments selected the remoter keys because they did not want the resonant, or open string tones associated with the more usual keys, or because it results different possibilities for modulation.