Depends on the crossover point. If you are using the sub to just augment the lowest notes (30 hz and below), then one sub in mono can sound very natural. However, if you are crossing over substantially higher--say 60 Hz or above, then 2 subs in stereo will sound more natural.
For 2 channel music I personally use 2 subs, but they are in mono and rolled in at 30 hz. The 2 subs are used to flatten out the room modes.
For 2 channel music I personally use 2 subs, but they are in mono and rolled in at 30 hz. The 2 subs are used to flatten out the room modes.