A 6" adjustable wrench is already in the on-board tool kit, used to remove cassette lock rings........as long as it opens enough to accept the removal tool and you don't needlessly crank down the lock ring.

Note: it does work to get minor dings out of brake discs with a thin rag or slip of paper to avoid scratches.

For BB5 and BB7 type mechanical brakes, if you have a business card and 5mm allen wrench, you can adjust your brakes on the road in about a minute. Try that before you think you need to bend the rotors.