Subs radiate in all directions because they are only extreme LF bass frequencies - it makes little difference that it is down firing or not...as long as stuff is not rattling (floorboards) or the sub is on an uneven surface and wobbling then you are probably ok.
Down firing is actually an advantage as you are less likley to damage the woofer.
Down firing is actually an advantage as you are less likley to damage the woofer.