Chad - I don't inject butts. I'm of the school that I can add flavor during the pulling process to same/better effect.
It provides for flexibility, I.E. creating say two different styles during the prep. Also you are not committing the whole roast to that flavoring.
Again - JMHO, to each his own.
I have injected buts on occasion but have to say in my opinion I didn't notice much difference at all if I simply added the injection/flavorings during the pull rather than for the cook.
That said, I would say most of the time I do add some Eastern Carolina Style to the meat when pulling, just to taste. It's a personal favorite and I get consistent positive comments. Then use a finishing sauce that includes butter, chicken broth (or stock from the butt cook), apple juice. The apple juice is typically frozen concentrate (not diluted) or cider if it's available.
I might even toss in a little of the dry rub that was used as well, depending on the mood or if the bark isn't adding enough of that.
Not enough smokiness? You can even touch that up during the pull with a little sprinkle of smoked paprika or chipotle.