The thing to keep in mind is crossover distortion. If your bias is set too low, one or both tubes in the push-pull pair will stop conducting before/after the zero crossing of the waveform.
Like all things tube, this is not a precise one-size-fits-all setting; it falls within a range. Dyna, for instance, picked a setting where they felt comfortable that the vast majority of EL-34s manufactured then would be "on" during the zero crossing, and stop conducting shortly thereafter.
The "real" way to set the bias, then, would involve a signal generator and a scope to observe the waveform at the zero crossing; amp manufacturers obviously could not rely on many of their customers being so equipped, especially in the '50s and '60s, so they developed methods to allow customers to get the bias into a range they felt would accommodate most tubes most often.
Interestingly, most guitar amps did not provide any user-accessible facilities to set the bias on their push-pull amps. Many did not even include a cathode resistor to measure in order to facilitate setting the bias on the output tubes. Some had no way to adjust the bias, even if you could measure it. They evidently relied on the consistency of the tubes being manufactured to fall within a usable range of bias, and didn't worry about the occasional outlier. Or, the resulting distortion they might cause.
