Whoa, this is interesting. I'm in a CoS game right now with a monk who is building toward a shadow monk/assassin multiclass. Since we're planning to play through level 10, my plan is to do Monk to 7, and then add 3 levels of Rogue at the end. Right now, I'm at level 4, and while sometime I kick myself for not taking a level in Rogue already for that sweet Sneak Attack and some extra skills, I don't really know where I'd dip earlier because IMO you kinda gotta take a monk to 7 or so. Extra Attack, Ki-Powered Strikes, and Evasion are all too good to pass up, at least in Strahd (where everything is trying to kill you and also magic).