IF a flight is less than full, seats further back are more likely to be empty. This goes for both regular and EMS seats. If you want to have an empty seat next to you choose a seat next to one that is currently empty as far back as possible.

As an example, the seats in EMS row 23 on the a321 non-mint are some of the best seats JetBlue has, with huge legroom but are much more likely to be empty than row 10. That said, you won't be first off the plane, which matters to some folks.