There have been lots of threads about this... but I'll go ahead and add the data points from my last trip that I checked bags on (earlier this month):

Arrival in GRU: all bags that showed up first were 'priority'
Intra-Brazil (FOR/BSB/REC/SSA/GIG): all bags that showed up first were 'priority'
Arrival in ORD: several priority bags showed up first (including mine) mixed with non-priority--did not stay around to see if more priority bags showed up at the end
Arrival in SFO: same as ORD.