Dunno, Obviously both approaches have benefits. I've done them both ways and both have turned out great. I dont think either approach is wrong or or worse than the other one. We are all probably arguing over the last 1-2% worth of quality at the most.
Our LAt flick (The Caller) was done in rec709. and all our deliverables (including film ones) passed QC with flying colors, not even one page worth of notes.
After all, didn't LightIron do Social Network on rec709 and it was nominated for a cinematography oscar, it cant be that bad? ;-)