More than okay. Hems that match are fussy and antiquated; it's far more modern to play with different lengths. I insist only that the coat extend far enough down to cover your jacket or any longer top (three visible hems are overkill). Some people sidestep the issue with maxicoats, but unless you're six feet tall, those to-the-ankle jobs look like bathrobes. I prefer a knee-length classic with crisply tailored shoulders, a slightly shaped waist, and a not-too-narrow bottom (to allow for fuller skirts).
Layer with flair—uh, make that flare—in a flyaway coat (Michael McCollom for Fleurette, $850).
Dress, Boss Black, $1,195. Brooch, Robert Rose, $48.