I'd have to confirm with my EE co-workers to be certain, but if your 14/2 wire is only rated for 15A then you would want to use a breaker that was rated for 15A (or less, assuming you could find a 10A breaker). The idea is to count on the breaker tripping before the amperage in the line gets higher than the wiring and devices downstream are rated for. With a 20A breaker on a circuit that uses wire rated for only 15A, you could exceed the wire's rating without tripping the breaker.