Modern dice tend to have rounded corners (as shown in the image here) and indented pips. Apparently this greatly reduces the amount of plastic required to manufacture the dice, and thus makes them less expensive. Turns out there’s a problem, though, this style breaks the randomness of the dice. They tend to come up as ones about 29% of the time (versus 16.7% if it was truly random). Think about that for a moment. The dice produce ones nearly a third of the time.
The main problem is the rounded corners. When that was fixed, the percentage of ones rolled dropped to 19%. Still too high, but much better. The indented pips were the cause of the remaining difference. When the tester used casino dice (square corners with non-indented pips), the percentage of ones rolled was “dead on” correct. And he also learned that casinos had researched this same issue and their results were similar to his.
So if you use dice for games and such, I’d suggest using a pair with square corners. And if you’re a hard-core purist, get a set of casino dice. (Unless you’re playing Axis & Allies, in which case you want a lot of ones.)