There isn't a whole lot you can do about how different browsers render subpixels, sadly. It'd be great if we could choose how the rounding should be handled through CSS, but nope.
Simply using pixels rather then percentages would solve the problem but that's obviously not what you want. I guess you could get around with static pixels if you change the values (possible to a percentage value) through media queries.
Whenever I find myself in a similar scenario I float
the last child to the right. A few additional pixels between the last and second last elements don't usually hurt as much as a few additional pixels between the last element and its parent.