I think flocking does help. The past few winters here in Colorado have been pretty mild. Alot of bluebird days when that sun is just a killer. The heads befor we flocked them would shine pretty good now with the flocking there is just a sheene. For what you have to pay for the flocking it is worth it. We have had our heads flocked for the last two seasons and I think it has but more birds in the bag. To do it right it is very time consumeing.
One hint:
For the flocking to last longer make sure you clean your heads and use a good plastic sealer before the glue and flocking and let flocking cure for a min. of 78hrs before you tap off execess.