Fitch,
As you probably know, everytime you open the door/lid you lose heat. However, I believe propane smokers recover their heat faster than wood/charcoal, so if you have propane you may want to try experimenting with spritzing ribs. It may add an hour or two to onto your cooking time, but if you aren't in a rush, it might be worth the trouble if you wind up stumbling onto something great.
Speaking of experimenting; since you are doing three slabs of ribs, maybe try spritzing one slab a every hour, another slab every two hours, and the last slab not at all. Do a little survey with your guests/family and divey up a sample of each rack of ribs to them and ask what they think of each? Take some pics and notes and come back and tell us your thoughts on the three? Just a thought...