If the long term plan is just a single sub, I'd keep the tuning around where Mike suggested, about 15hz. If you ever plan to build a second, I'd tune a little lower, about 12-13hz. Not a huge difference either way.
You should be safe with either tuning in regards to low frequency effects. Just don't run continuous sine waves through your sub below tuning at spirited volumes. Really, I don't know why people run continuous sine waves at all - RoomEQ does a sweep, and that yields a much more reliable FR measurement.