It follows a pattern, but it is effectively randomised by the actions of everyone else checking in at different times. It is deliberately intended to provide a random result. The pattern isn't as simple as working from back to front or vice versa, it hops back and forth between rows. So if this is important to you, you best pay.

Note that at most (but not all) locations, FR embarkation and disembarkation is done at both front and rear of the aircraft, so if your concern is about getting off quickly then rears seats can be better.