CAD models are balanced to <1% volume for each face. This is done by changing font size and depth at each face to match the others. I have quantified this. Some designs have more liberties taken to accommodate the aesthetics. The engraved set is perfectly balanced in CAD. The Amtgard set is the least balanced (still <1% variation though).
I run an NSCNC MIRA-3 5-axis CNC mill specifically designed for accuracy. It has resolutions of 0.3microns.
In addition to this all dice designs go through a series of rolling tests and show no detectable inbalance after hundreds of rolls.