Well, Matt, if you're going to foil, you would do so when the internal temperature of the butts is at about 165°F and then cook them until they reach your desired level of doneness. Once that has been reached, you would rest them for as long as necessary before you need to pull them apart. If you rest them in a dry cooler or Cambro-type device, you can hold them for many hours without worrying about the meat temperature dropping into the danger zone below 140°F.
If you are not going to foil during the cook, you should plan to wrap them up for the rest period.
Regarding rotation, I think you are going to find that the lower grate will run a bit cooler since you are planning to run with a full water pan. The lower grate would likely run hotter if you had an empty, sand filled or clay saucer filled pan. If you plan to foil, rotate them about 3 hours or so into the cook as I'm guessing it will take about 6-7 hours to hit 165°F at your listed cooking temperature. If you are not planning to foil, I don't know that I would actually bother with rotating them. They'll be done when they're done!
