IME, just before the 48 hour mark, Jetstar automatically allocates seats to every passenger on a flight, whom has not already purchased a preselected a seat.
Thus at T-48, when WCI “officially” opens, the number of seats that are not filled will match the number of seats left for sale on that flight. So an empty seat (at less then T-48) will remain empty unless - 1) Jetstar sells another ticket, and said passenger selects that empty seat, 2) Another passenger changes (or is changed at the airport/onboard) from their seat allocation to that seat, 3) A standby passenger gets on that flight at the airport. (Beware that on certain routes/times, there is heavy demand for standby (i.e. staff) seating, so “empty middle seat only” rows are more likely to fill on those flights.)
If you WCI for your return flight at the same time, and it is more than 48 hours prior to that flight, the system will not have pre-allocated a seat to every passenger on the aircraft yet, so only those passengers who have pre-purchased a seat allocation, and all premium seats (i.e. upfront & exits rows, if you didn't pre-purchase either option) will be blocked out, giving you a whole lot more choice of where to sit. (But obviously this seat map will not be indicative of what seats are likely to be filled or remain empty after the T-48 system allocation. However, generally the system allocation does this fairly methodically from the front to the back, so "empty middle seats only” rows toward the rear, are more likely to remain empty than those at the front, if a flight is not fully loaded.)
Jetstar uses front and rear boarding/disembarkation at most domestic ports, but often the last flight into an airport of a night (i.e. when the aircraft is not going to turn around and fly out again until the following day) will be forward door disembarkation only. (So if you select a seat at the rear of the cabin, you will be last off the aircraft.)