In the past, and for the most part now, I do not spray. I prefer to leave the lid down, and I prefer to have moist air in the cooking chamber throughout the cook. I do not believe that opening the cooker aids in this goal, as the hot air will rise and evacuate the chamber the minute I open the cooker. This causes variations in cooker environment, the more often you do it, the less consistent your cooking is. This leads on unreliable results. I set chamber moisture by use of water pans or water cans to evaporate moisture into the chamber, which will prevent surface drying.
That being said, I have been messing around with a new spray on sauce, which does require I open the chamber, and I have been getting drier ribs as a result, with notable hardening of the bark, which is counter intuitive, but, was exactly what my experience suggested might happen. I still am working with that spray on sauce, as I believe the results in terms of taste and appearance are superior. But, there is also no doubt in my mind that the opening and closing is creating changes in chamber moisture and temperature in my small cooker that are not acceptable at this time.
In a larger cooker, at lower temperatures, with more thermal mass and a lot more meat, this may not be an issue, as the moisture and heat are more effectively controlled by the thermal mass of the metal and the amount of meat a larger cooker can hold. Since I cook in a kettle, getting even a little out of range can cause me to have to chase temperatures, not a good thing.