This is one of those brewing debates that lacks profound scientific data. Lets say your recipe calls for a 5 minute hop addition. At 5 min you add the hops to your hop bag, and 5 minutes later you turn off the burner. Now say you leave the hop bag in, but wait 3 min to start cooling...you've technically just added 3 min to all of your hop additions and your 5 has become 8. Not a huge deal but when trying to repeat recipes, maybe it is.
Similarly, one can argue that even if you start cooling RIGHT away, the hops are still being utilized to some extent. Depending on how advanced your cooling method is, you could still be at 200 degrees after a few minutes.
Because of these unknowns, I pull the hop bag at flameout.