There are certain temperature thresholds:
131 F for 3 days
151 F for 2 days
160 F for 24 hours
165 F for 12 hours
We use the chimneys to ensure the hot middle does not get past 170 F so the hot middle can properly kill pathogens for the appropriate time frames listed above.
When doing chimneys before the first turn you run the risk of losing too much moisture, cooling the pile down too much or even burning up too much of your N too early in the pile's creation.
Awesome info. This winter I completely abandoned our compost =/ gotta start from scratch as soon as Spring arrives.