On widebodies, especially if they seem very full during OLCI, I often try to snag one of the seats in the back just after where the configuration goes from 3-x-3 to 2-x-2. On some airplanes (eg. most 747s), the extra "half a seat" to the side and in front comes in very handy for stretching my legs, keeping carryon in reach, etc.

But on short-hauls, tight connections or destinations where it's important to get a head start on immigration queues, I prefer as far up from as possible.