I always inject pork butts (no special reason, just think it adds flavor), but smoking butts without injection I'm sure can also produce an excellent product.
I inject the night before, and put a light coating of rub (which the down side of the meat will mostly come off because the liquid leaks out during the night). When I pre-heat the smoker, the butt gets another light shot of injection. From here on you have to gently handle the butt or a lot of liquid will leak out. I put tangy types of mustard on, then the rub.