Even if they were to get some frost damage this early, the worst that would happen is it would kill the apical meristem (growing tip), which would actually be like pinching & cause more branching. I always pinch my hardy hibs hard to increase the number of blooms. When they're 12-18" tall. I pinch back to the lowest 2 healthy leaves, which increases the number of stems considerably and makes the plants sturdier.

Al