From what I have seen flying the red eye between LAX and MCO and back a couple of times and once from SNA to DEN and back (so six flights, YMMV), the entire plane was sold out except for on two flights, where there were maybe 5 or 6 seats on the entire plane that were empty. I think that Frontier aggressively prices its flights to make sure that every seat is full. I would not count on any middle seat being empty.

One thing I did notice was that as the check in time came around and seats were being automatically assigned, the seats in the back of the plane (save for the last row which I believe is kept open until the very last minute to seat families with small children together if needed) were assigned first, working its way up to the front. Because of this, my guess is that you would be more likely to have an empty middle seat towards the front of the plane than towards the rear. However, again, I wouldn't count on this because every flight I have been on has been mostly or completely full.