If the impulse drive is a pure "field drive", and uses one of those ubiquitous rows of doughnut coils, it probably can work pretty much as well forward and aft, but might have limitations in other directions. If the drive is an "augmented rocket", then there's bound to be a bias towards forward flight since the nozzles always point aft (or aft and up - what's with that?).
Going by onscreen evidence is probably our best bet here. And we see starships usually maneuver in a manner consistent with vectored rocket thrust, and inconsistent with the free 3D movement that an omnidirectional field drive would allow. So that's one for the "augmented rocket" theory - and it would seem to carry the lemma of thrust limitations in certain directions.