Personally, it depends on the meat. I don't waste mustard on meat. Tried once, went back to olive oil. Regardless of meat, I pat the meat dry with a paper towel, spray with olive or canola oil, then LIGHTLY apply rub. I prepare all my meats like that.
Beef of any kind, plus pork loins/tenderloins: rub is applied 12-72 hours before smoked. My schedule determines the time.
Pork butt, poultry, spare ribs, country ribs. Apply the rub right before the smoker.