i think it depends on how low u go. if ur plannin on goin lower than like....1.7 or so, then u should probably get the front too. im not sure if its absolutely necessary, but i guess its always safe to.
just take a look at your camber after you set your springs. if its more than -1 deg, i would def get front camber for the adjustability. i think it would take at least 2" front drop to go that far, though.