I use a rub and serve BBQ sauce on the side after it is pulled. I don't like forcing sauce on anyone.
I do not inject, pork butt has a lot of fat in there to keep it nice and moist.
Here is the formula I use... other's vary theirs slightly. Some people say 60% yeild on the cooked weight, I say 50% to make the numbers easier and leftover pork is a good thing. You lose weight due to cooking, moisture lost, and the bone in the middle.
(Pre cooked Wgt) x (yield) / (portion)
8 lbs x 50% yeild = 4 lbs cooked meat
So 4 lbs cooked meat x 16 oz in a lb = 64 oz
So now for portions. I use 4 oz for sandwiches or 6 oz for plates. If you are feeding a bunch of football players, you of course want to increase your portions. If you are serving a bunch of sides with it, you can have smaller portions.
64 / 4 oz sandwiches = 16
64 / 6 oz plates = 10.6
Also, the reason I say 50% is easy math is because if you are using 50% and 4 oz sandwiches, you just double your precooked weight
We cook PP for sandwiches a lot for charity and such.
All leftover pork should be cooled and frozen and heated up anytime for BBQ without the hassle.