Are you speaking of ribs? If so, you can paint on some sauce towards the end of the finish line...say 15 - 30 minutes, tops. Any earlier, you run the risk of over-caramelizing the meat due to the sugar content of most sauces. Concentrate on seasoning the ribs (or any other meats/poultry) with a seasoning rub, which will add depth of flavor during the cooking process.
Large cuts of meat (brisket & pork butt) will benefit from a "mop" during the smoking process. Think of it as a marinade that's applied to the outside of the meat.
Back to ribs...my suggestion is to finish them without a sauce and serve one on the side when you're ready to eat. This allows you taste the meat in its purest form.