Is there a need to trim some of the fat off or a pork shoulder. That is one thick fat layer with skin.
it doesn't seem like rub would get through to the meat.
The skin should be removed and I only mess with trimming fat if it is more than 1/2" thick. Remove the fat and Smoke the skin, it can be used to flavor a variety of soups and beans, made into Cracklin's or the Dog will love you for the treat...JJ
I try to trim as much fat as I can off of my shoulders. There is plenty of fat inside that will have to render as well. I usually get compliments from people that are eating my PP for the first time about how there's not a lot of fat in it. I don't think I ever recall anyone complaining about it. Besides that way you can get more rub close to the meat.
I also usually pick out any fat chunks that I come across when I am pulling it too. I also take the the juice from the foil and dump it into grease separator to remove the as much grease and fat as I can before pouring it back on after I pull it.
Hope this helps