You have me there, sir, and I'm mad at myself because I had code for that in place, then removed it.
But humor me this - should a multi-slider "round" differently for a multi-slider? That is, flooring (then perhaps correcting up) for the bottom thumb at some precision and ceiling-ing (and correcting down) for the Nth thumb at the precision? I believe that is where I'm really seeing the problems I will have to work around.
I should be able to work around it, but I'm wondering if slider rounding errors is so common a problem that it merits further discussion (I can move this off the bug forums if you want, Scott).