I cool the wort to pitching temp before transferring to fermenter and before putting the fermenter in the bag. The cool brewing bag does best at keeping the wort cool. It is not as good at cooling the wort from a higher temp. It can be used to cool the wort but it is much slower than a wort chiller or ice bath.
Ah yeah.... I didn't intend to imply to use the bag to cool the wort to pitching temp... I just meant use the bag from the start of fermentation on. Don't wait until it's already fermenting to try and lower the temp as it will be too late at that point.