you will find different opinions on many areas of hatching as it is both an art and a science. Its one of the great things about byc and thought sharing.
Ron has great personal experience and has been awarded a badge of honor for his education services here,
Ron and I agree on most things but differ on time to start turning. He is right about the small things being made a big deal of in sally's thread. There are several of us who spend hpurs researching scholarly articles with statsitically significant data on ways to tweak the hatch to get just one more egg to hatch.
This is the background for my opinion
I have received around 2000 shipped chicken eggs and have never seen an egg with truly detached aircell hatch. I have seen an increase in dead in shell chicks through not turning eggs after a 12 hour rest at temp. The increase in dead in shell chicks increases proportionately with time not turned. To sacrifice the good of the other eggs is not worth it
Turning of the eggs will facilitate position of developing embryos and ensure that nutrients are evenly distributed for embryonic development. When the yolk is the largest is when turning is most important.
Retardation effects on the development of the chorioallantois, absorption of albumen and growth of the embryo occur with delayed turning.
Delayed turning eggs hatched later than eggs which were turned throughout incubation.
and the list ron talks about up there ---
http://msucares.com/poultry/reproductions/trouble.html - look in the
Pipped eggs, but died without hatching section
also