Rmd-
When you wrap the butt in heavy foil and put it in a cooler with towels to cover it, it will cook another 10 degrees or so. This would make it about 200 degrees or so. If that is what you want, go for it.
I pull mine at 185, and let them sit wrapped up so that they get to about 195 degrees. We all get into our little routines in this BBQ game- and, for the most part, it is what works for you and the people that are eating your product that are most important in deciding on the little details that make this "hobby" as much fun as it is!