I was taught to mop and then wrap in foil to finish.
Lately I've been reading a lot here where folks don't mop and foil, and it's supposed to be great. So the last butt I did I didn't mop or foil, and it was MORE moist and tender than all the other butts I've done. And the bark was better too.
I use a Weber kettle with the smokenator, which cooks in smoke and steam. Don't know if you aren't using water what the results will be. However, I've read enough here to convince me that if you don't mop, your dinner won't be ruined. So you may as well see which way you prefer.