I foil when I take them off the grill to rest for and hour or as long as needed before serving. I did two butts last week and pulled at 190 and 196 respectively and foiled for an hour with a bit of apple juice/cider vinegar in the foil. The butt pulled at 196 was MUCH easier to pull than the other one. I usually cook at 220 and start them the night before, just to make sure they are done in time for company.
Blues, BBQ and Homebrew!