You can take butts all the way to 200-205 w/out any issue. You just might want to slow the process down to get some more smoke on them. Don't worry too much about internal temp. It's a good tool, but it's all about tenderness. When the bone is loose and seems like it'll slip right out, or when you put it a fork and twist and the meat just comes apart, you know you're there.
Like others have said, pork butts are extremely forgiving and really hard to ruin.