The 20-25x rule was basically a ballpark guess that people used to determine the maximum voltage to use, as there was no simple formula until gecko came out with the one below a few years ago.
And if you look at gecko's website, you'll see the 32 x sqrt(mH) is the maximum voltage. They don't say it's the optimum voltage. Keep in mind that while the motors won't overheat at this voltage, they can still run very, very hot. And in a lot of cases, the maximum voltage is not always required.
It's always been my understanding that the ideal voltage you need is the lowest voltage that gives you the performance you need.
Say the formula tells you the max voltage is 48V. Now, lets assume that 48V allows you to run your stepper at 1200rpm, but you never spin it faster than 800rpm. In that scenario, the optimum voltage would probably be closer to 36V. That will allow your motors to run much cooler, but still deliver the desired performance.