Originally Posted by craigthemif

My general theory (with only short haul flights to test my theory) is that automatic seat assignment algorithms are designed to:
1) do their best for customers with elite status
2) do their worst for non-status customers, so that they are inclined to pay for a better seat assignment now or in the future

Seats are generally allocated back to front, although weight distribution requirements means that this isn't a hard rule.

If one of the back rows has an empty middle seat and the flight is going to be reasonably busy, somebody without status will be allocated to it and I don't think that time of check-in ultimately makes much difference.

The only surefire way to get an aisle or a window is to pay for it. (treat it as a small insurance premium against a truly horrible seat)

To increase your chances of an empty middle seat, then you should pay for a seat with an already allocated single traveler in it, as close to the front as you can. Therefore you know that nobody is going to come along and pay for the middle seat, and this middle seat might be informally blocked if the other passenger is elite. But when the flight is full that seat is going to be taken anyhow, and hopefully not by one-half of a couple who will then be begging for you to change seats.