where I work it depends on the airlines policy. If the airline uses actual weights, all bags are weighed (both bulk or containerized). If the airline uses standard weights, all bags are counted.
As for the accuracy. It sometimes depends on good judgment. If you see you load heavy samonites or very light attache cases it might be a good idea to use either te reported bagfigures from the ramp or the figures you get from the checkin-system.
Large discrepancies between rampnumbers and checkin-numbers might indicate that you are still missing carts or containers still at the bag sorting area