Just another data-point: on a flight BSL - HAM I checked in 21 days before departure, on a ticket (single ticket, no speedy boarding, no luggage). Seat map showed a plane that was approx 60% empty. front part was well filled, and the back half was completely empty. I took a gamble....
.... and got a "B" seat in row 21. grrr...
It's a short flight, so not too tragic, but also not fun. I'm tall. I think I should have waited until closer to flight date. Learned my lesson.