It depands on the airline. Some airlines will block seats that are not available for pre-asignment or that you can only pre assign on their system, on other airlines it is a true representation of seat availability (as much as a seat map can be). The US airlines seem to have all seats open for assignment, hence from my experience it is pretty accurate for them, although I have an upcoming flight with AF and it certainly isn't showing all available seats as witnessed when looking at the seat map on check my trip.

The new feature where you can view available seats without going through the booking process is a good additonal feature but beware it will not show premium seats as open. So for example a LGW-CLT flight on US will show all the seats the airline reserves for *A elites as occupied. However if you go to complete the booking and enter your FF number then these seats will show as available so for total accuracy you need to do a dummy booking and enter your FF number. Same for UA E+.