Do airline dispatchers pre-block seats for weight and balance on large wide body aircraft?
Yes sometimes, because with most aircraft you can't fill all of the seats and all of the fuel tanks and stay under max gross weight, meaning the airplane can't achieve its potential maximum range with a full cabin and on long range city pairs some sort of passenger/fuel horse trading may have to go on.  

So on flights where the airplane needs more range that it has with a full cabin, it may be forced to limit seats filled to accommodate the extra fuel, and it will distribute the empty seats throughout the loading zones to prevent load concentrations for balance reasons.

You will also see seats blocked off to accommodate dead heading crew, but if you see a significant number blocked off in each cabin loading zone, and the seats are still empty when you board and its a long flight, it was probably due to fuel requirements.