Bass is basically omnidirectional so doesn't matter if down firing, or standard front firing. the bass will radiate in all directions.
@kofibaffour This isn't exactly true. What is true is by the time your ear can sort out what bass note is being played, the bass has bounced all over the room and that is why its omnidirectional.
But it does bounce. You want it to bounce as much as possible to reduce standing waves which can cause a boom or a loss at certain frequencies and locations in the room (IME at the listening position...). For this reason a down-firing driver will be a bit more effective if the cabinet has only one driver.
If you have only one driver in the sub and its not down-firing, I've found it more effective to point it at an angle towards a nearby wall, so the bass energy has bounced around a bit. This helps reduce standing waves.
If the cabinet has outputs in two directions angling it is even more effective.