Is there any camo benefit to applying it to the gun on your tank?

DomoSapien: To echo what others have mentioned, it is indeed only required on the hull; this hasn't changed.
So if you want to stretch the most value out of your camo set (say you got some camo in an event, and it's actually camo instead of a style) you can just apply it to the hull for each season so you aren't burning extra camo patterns on making your gun etc. look fancy.