If you crisp the ribs you can apply your sauce after they come off the high heat. You can put on the barbecue sauce and return the ribs to the smoker for a while to help the sauce sink in. However, don't leave the ribs here too long. The combination of the low and slow heat and the barbecue sauce is going to soften up the surface of the ribs again and you will loose that crispy surface. You can simply apply the sauce, cut and serve if you want.
If you are not going to be crisping the surface of the ribs you can start applying the sauce anytime towards to the end of the cooking time. If you wrapped the ribs you can start putting on the sauce as soon as the foil comes off. By putting the sauce on and continuing to smoke the ribs you will get more smoke into the sauce and the barbecue sauce will cook onto the surface making the ribs sticky, but not dripping in sauce (if you want them dripping keep adding sauce). If you want the extra smoke flavor put in more wood.
