Some people swear by it, others don't use it at all. "It just collects dirt and causes oily feathers!".
Some things I could recommend, be sure the coop is not holding humidity. Ventilation is needed (Ventilation should be coming from above perches, not at roost level)
humidity causes frostbite in alot of cases. Putting in a heat lamp in the coop will help as well. We dub our birds to prevent frostbite. Frostbite can cause infections, infertility,
and possibly death. We cannot absolutely PREVENT frostbite with outdoor birds. Any temperatures below freezing - birds could be at risk of frostbite. I personally, and this is
my own opinion - do not believe in the vaseline method.
Thank you for all of the advice. I live in Mass., and it does get really cold. My coops do have ventilation, and i refuse to put heat in them. I've thought about it, but after reading up on it the risks are too great. I couldn't deal with the coops catching fire, or the lighting malfunctioning and my birds freezing to death because they have become accustomed to the heat. I believe that their body heat will keep them comfortable.
I just rescued a abandoned rooster (leghorn type) with a large comb who has pretty significant frostbite on his comb AND wattles--and I live in the Southern Arizona Sonoran Desert! It almost NEVER gets below high 20's here and then only at night and for short periods of time (1-3 days in a row). Apparently here, if they have absolutely no dry shelter at all, it can even occur! "Grease" does help prevent frostbite. Bag Balm is often applied to milk cows teats to keep them from freezing in cold weather. Historically, seal/whale fat and bear fat were applied like lotion to keep skin from freezing in Alaska and northern states.