Used to be that I just painted on the mustard before applying the rub. After watching Mad Momma and the Kid on the Food Network massaging the meat at the Jack Daniels, I started doing that as well. I find the rub adheres more uniformly, and I also use less mustard.
Never really felt that mustard adds much in flavor, having tried both with and without side by side and, like stated before, unable to tell the difference.
I do not apply mustard to ribs or chicken, sometimes to brisket.
One thing I do use mustard for is a finishing glaze. Equal parts mustard, maple syrup and brown sugar, heat to just beginning to boil, and slather on meat in last hour of cook. Seem to be able to taste the three flavors on the meat, including the mustard. Not "hot dog" intense, but just a hint of it. Works especialy well on ribs. Note-the glaze should be watery when applied.
Dale