If outdoors or in a very large indoor arena, group the subwoofers together. If you can keep them close enough together (within 1/4λ), they'll act as a single acoustic source.

If indoors, spread them around to average the effect of room modes. Reflections make virtual sources that cause unavoidable cancellation nodes. But by spreading the subs, you can make the interference so dense that it tends to average the sound field throughout the room.