Six-sided dice traditionally have opposite faces adding to 7, and this tradition is carried over (unquestioningly, it seems) to other dice. Is there a good reason for it? Why wouldn't you instead seek to make opposite ‘hemispheres’ as equal as possible, ideally arranging that the center of gravity of the pips is the geometric center of the die?
Added four years later: I recently did some searches: There are no ideal arrangements for D4 (obviously), D6, D10, D12; three arrangements for D8; 876 arrangements for the rhombic dodecahedron; and somewhere between seven and gazillions (the search is a long one!) for D20.