It's quite hit-or-miss, but exit rows sometimes do get boarded early--it just depends on the GA--some do it, some don't. The rationale is that the early boarding allows time for the FAs to go over the safety briefing with the exit row pax and get the verbal commitment that they are willing to assist in an emergency. In practice, even when boarded early, I've never had an FA do the exit row briefing until the plane is fully boarded (or at least very close).