Only problem with injecting pork butt is that once you penetrate the muscle you need to bring the internal temperature up to 135 degrees within 4 hours to be safe. One way around that is to allow the outside of the piece of meat to achieve safe temperature and then injecting it after insuring your needle and marinade are sterile
That said many members will sterilize their needles and marinade and inject prior to putting the meat in the smoker. They practice impeccable food handling and do their best to speed up the amount of time the meat is sitting in the danger zone (41 to 135). We have had many discussions about this on the forum and now that you are aware of the potential problems you can decide what to do.
Normally we don't inject BBQ sauce. Any number of combinations of apple juice, soy sauce, beef or chicken stock, vinegar are common. Personally I think one to the things that make BBQ tasty is the way it carmalizes when put to flame. Why not use the BBQ sauce as a mop and then mix a bit into the pulled pork or on the side at dinner time?