Many of the stickburners wrap their butts and briskets because it's easy to oversmoke when cooking completely with wood. When cooking on our offset exclusively with wood, I'll wrap butts and briskets when the color is to my liking, which usually winds up to be about 165 or so - as ron said, though, doesn't matter exactly what temp 'cause it's going to keep cooking anyway, and it'll cook faster once it's in the foil.
I personally don't foil butts OR briskets on the WSMs at all, unless I'm having to try to speed up the cooking session for some reason.
Keri C