We're starting to get into some pretty meaty issues, but popcorn is a very important resource. We use the word popcorn to refer to any low level monster that you can harvest at little or no cost in resources. Exactly which monsters are popcorn depends on your character and situation, but because you typically begin engaging the boss at around level 7, 7s, 8s and 9s and almost never popcorn (Assassins not withstanding). The best way to get them is when you are still low level so that you can recover via ding. Once you're on the cusp of level 8 (or maybe level 9, if you decided to start on the boss at 8), it's too late to get those high ranking monsters. There are 2 9s, 3 8s and 3 7s on the board. Not counting bonus xp, that's 63 xp that become lost if you hit the high levels too quickly.
The best case scenario is that you start on the boss right before hitting 8, with all of the monsters at 7 and above already dead and as many of the lower level monsters still alive as possible. Done right, you can pull three dings in a row while fighting the boss. The Balanced Dagger is doubled edged because it causes you to hit higher levels without clearing the 'ranking monsters,' and does so at the cost of popcorn. This can sometimes get you stuck in situations where you are high level, but can't possibly take the boss on one load out of resources and can't get any more level ups because all of the remaining monsters are too tough for you to fight at a profit.
I'd say the dagger works better with regen fighters. They tend to prize black space more highly than 'soft' xp. The Monk can be very good with the dagger because he has natural difficulty harvesting popcorn and needs lots of black space left over for fighting the boss, which means that he can't count on finding all of the ranking monsters in time.