What is the margin of safety with selecting circuit breakers?

The NEC requires that if the intended rating of a circuit corresponds to a standard size breaker then that size breaker must be used. In most instances, "rating of a circuit" means the intended connected load plus 25%.

If the intended rating of a circuit does not correspond to a standard size breaker, you can go up to the next standard size without any other adjustments.

In most circumstances, besides what I've already stated, a 14 gauge copper wire cannot be connected to a breaker larger than 15 amps, 12 gauge copper to a 20 amp breaker, and 10 gauge copper to a 30 amp breaker.